Can you think of a word that is like a vicious circle, i.e. where it's difficult to say which thing creates the other, but not in a negative way? I know vicious circle also means that but it has a negative connotation.
Classic chicken-and-egg situation.
chicken-and-egg ADJECTIVE [ATTRIBUTIVE] Denoting a situation in which each of two things appears to be necessary to the other, making it impossible to say which came first.
A vicious circle is a negative feedback loop (wikipedia link).
A classic example in business includes missing an important deadline. Managers express their disappointment with the employees, and morale drops. With the lower morale, productivity drops. With the lower productivity, the next deadline is missed. Ad nauseum.
So, we need to find a way to make that into a positive feedback loop (wikipedia again).
Recognize the situation and re-organize our task schedule so we can meet the next deadline. Praise employees for their dedication and thereby increase morale. Watch happily as productivity soars and we smash the following deadlines.
If you need a completely neutral term, then that is just simply feedback loop (once more for wikipedia).
A self-perpetuating situation, that is, a situation which by its nature causes itself to continue to be.
The term "positive feedback loop" may fit your needs. It refers to a process that is self-enforcing, like the greenhouse gas effect. The opposite is a negative or "stabilizing" feedback loop, where the process is self-stabilizing, like a thermostat.
The positive complement of a vicious circle is a virtuous circle
The terms virtuous circle and vicious circle (also referred to as virtuous cycle and vicious cycle) refer to complex chains of events which reinforce themselves through a feedback loop.1 A virtuous circle has favorable results, while a vicious circle has detrimental results.
The word uroboros (sometimes spelled ouroboros) is defined by oxforddictionaries.com as
A circular symbol depicting a snake, or less commonly a dragon, swallowing its tail, as an emblem of wholeness or infinity.
From this, some writers have referred to an endless cycle as an uroboros.
When you say, circular, "where it's difficult to say which thing creates the other", you might say
self propagating or self replicating.
This has a slight problem if you're trying to make a distinction that there is more than one thing involved. I'd wonder though, if either one can create the other, and vise-versa, ad infinitum, how distinct could those two things really be?
Similar to some other answers, but for a term that just matches your statement "like a vicious circle", but less negative, you could say:
Symbiosis: Negative and Positive
If you are adamant in wanting a single word to describe the phenomenon you describe, I suggest the word symbiosis [Greek sumbiōsis, companionship, from sumbioun, to live together, from sumbios, living together: sun-, syn- + bios, life], which describes a relationship between two organisms in which one is helped and one is hurt, or both are helped.
A negative kind of symbiosis is parasitism, in which the host supplies the parasite with an obvious benefit while only the host suffers. A tapeworm is such a parasite, since in the gut of its host it deprives the host of needed food and nourishment. The parasite gains while the host loses.
On the other hand, a positive kind of symbiosis is mutualism, in which both organisms benefit from their relationship. The bullhorn acacia tree and a species of red, stinging ants, for example, enjoy such a relationship, with the tree being the host, and the ants being the symbionts.
The tree provides ants with a home and with two food sources, while the ants, being naturally territorial and aggressive, attack the tree’s natural enemies, such as herbivores (grasshoppers, caterpillars, deer) and even other competing vegetation! Dare I invoke that oft-used phrase, “win/win situation”?
The "App" To a Beneficial Cycle
Here, I suggest, is where the notion of a beneficial cycle is applicable. Instead of a vicious cycle, there is a healthy interaction which, because of its mutuality, repeats—or cycles—itself again and again.
Reinforcement and Conditioning
Another word which describes a cyclical phenomenon which can be either negative or positive is reinforcement. Positive reinforcement, of course, is associated with a reward which tends to encourage--or cycle--desirable behavior, as when a teacher, for example, praises students for giving good answers. Chances are, the teacher’s praise will boost students’ answer-giving behavior and at the same time set the stage for the students feeling good about themselves and perhaps even doing better in all their curricular activities.
Punishment, on the other hand, is a negative kind of conditioning which tends to extinguish unwanted behaviors, whereas negative reinforcement (also called escape conditioning) is a special case of reinforcement through which non-desirable behaviors are actually encouraged. A good example of negative conditioning is a parent who lets a son or daughter stay home from school because they have a test that day. The parent is simply encouraging future avoidance behavior in the child.
Cyclical phenomena are all around us, and they can be negative, positive, or combinations of both. A recent strain of gonorrhea, for example, is proving to be increasingly resistant to traditional anti-bacterial medicines. That resistance can be described quite accurately as a vicious cycle, since bacteria (and viruses) are adept at adapting to the challenges medical science throws their way, and they become increasingly stronger and medicine resistant.
By the same token, however, the human beings in a given community can engage in cyclical behavior of symbiotic mutualism, two examples being "random acts of kindness," and "paying it forward"! Now those are the kinds of cycles which encourage both surviving and thriving.
Catch-22 is slighly negative but less so than vicious circle while also conveying unclarity about cause and effect or which thing creates the other
The idiomatic metaphor "vicious circle" means literally "a vicious or very difficult path, upon which one following to the end will find oneself back at the beginning". Its metaphorical uses, though, can overlap with some other idioms:
Stuck between a rock and a hard place - colloquial/general use - This idiomatic metaphor describes a situation in which one might be stuck, with no option offering an opportunity for progress. Example: "With a deadline fast approaching and no sources in mind to help him build his story, Josh felt stuck between a rock and a hard place."
One step forward, two steps back - colloquial/general use - Another idiomatic phrase, this one describes a sense of frustration over an inability to make progress, usually in a situation where the solving of one problem has created or will create other problems. Example: "Every repair we make is one step forward, two steps back! We can't fix the ship without the proper tools!"
Stuck in the status quo - colloquial/professional - Not so idiomatic, this phrase establishes that there is a status quo (that is, a "way that things are", as though that way is persistent and conveniently describable, since everyone should be familiar with it) and that the speaker is unable to break out of it or is unable to deviate from the norm. Example: "When Jessica's proposal for a morale focus group was shut down by company leadership, she grudgingly derided the company as stuck in the status quo."
Caught in an endless loop - colloquial/general use - Probably the closest in meaning to the idiom mentioned in the question, but with less focus on harm to the speaker, this idiomatic phrase describes a sense of being forced to repeat the same task or event over and over again. While quite similar in structure to the idiomatic phrase used in the question, I'm including this one because it is used colloquially. Example: "'You were denied for a promotion again?' 'Yes... Sometimes it feels like I'm caught in an endless loop..."
Stuck on a/the hamster wheel - colloquial/informal - This is a cuter form of caught in an endless loop. It means the exact same thing.
Weighing in very late on this one, but the phrase (make that proverb) that comes to mind is "What goes around comes around":
The status eventually returns to its original value after completing some sort of cycle.
A person's actions, whether good or bad, will often have consequences for that person.
Life, in a way, is a neutral "vicious circle": if good goes around, good comes around; if bad goes around, bad comes around.
Karma is a related concept:
karma : the force created by a person's actions that some people believe causes good or bad things to happen to that person.
Good actions by a person cause good things to happen to that person; bad actions by a person cause bad things to happen to that person. In other words, what goes around comes around. :-)
A phrase that I haven’t seen in the answers yet is cycle of dependency.
A cycle of dependency is essentially the same thing as a vicious circle, but it’s a term often used by people who want to break the cycle and help others live better lives.
The word vicious is derived from vice, and a vicious circle is therefore somewhat associated with sins and sinful acts. However, many circles of dependence arise from helplessness rather than ill-intent, as when children growing up in poverty find it hard to overcome the disadvantages that were forced upon them.
Circle of dependence does not have the same moral overtones as vicious circle, and in most cases would be a “less negative” expression.