Is there an idiom for "People quickly get used to good things"?
"take for granted"
would be appropriate. From Oxford Dictionaries:
Fail to properly appreciate (someone or something), especially as a result of overfamiliarity. ‘the comforts that people take for granted’
Might be unfamiliar, depending on your audience, but hedonic adaptation (or 'the hedonic treadmill') is perfect for this. Wikipedia:
The hedonic treadmill, also known as hedonic adaptation, is the observed tendency of humans to quickly return to a relatively stable level of happiness despite major positive or negative events or life changes. According to this theory, as a person makes more money, expectations and desires rise in tandem, which results in no permanent gain in happiness.
You never miss the water till the well runs dry.
Prov. People are not grateful for what they have until they lose it.
Jill: I never realized what a good friend Jeanie was until she moved away.
Jane: You never miss the water till the well runs dry.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
There's a proverb:
Familiarity breeds contempt
Extensive knowledge of or close association with someone or something leads to a loss of respect for them or it.
There's a saying "What have you done for me lately?", which was around long before the 1986 Janet Jackson song. See for instance this 1977 newspaper article, which says in part:
In politics, as the old joke explains, power depends upon being able to give a heartfelt answer to the question "What have you done for me lately?" It is now the bureaucracy, not the [political] parties that grants critical grace and favors.
As parties shrink, lobbies take over, The Wilmington Post, December 31st, 1977
See also, "All right, but apart from the sanitation, the medicine, education, wine, public order, irrigation, roads, a fresh water system, and public health, what have the Romans ever done for us?"
According to Garson O'Toole on the Linguist List, this goes back to a joke about bankers first printed in 1943.
There is a set phrase in U.S. divorce law to describe the expected level of support (in the form of alimony in some states, and through a suitable division of the couple's "community property" in others) of the nonworking spouse following the dissolution of a marriage:
"the lifestyle to which [one] has become accustomed"
Although I'm not sure how widely known this phrase is outside legal circles, I've heard nonlawyers use it on multiple occasions. A cartoonist named Sam Hurt used a jokey play on the phrase in his comic strip Eyebeam, in connection with a law student character who was mapping out the trajectory of his career after law school: "the lifestyle to which I would like to become accustomed."
In any event, the phrase is used idiomatically to refer to a certain level of affluence and comfort claimed as an entitlement—never to a level of straitened circumstance that one has grown accustomed to enduring—and in that respect implies "good things that one has gotten used to."
You could express something like this by starting:
How easily we forget...
Too easily we forget...
Give them an inch, they take a mile.
Make a small concession and they'll take advantage of you. For example, I told her she could borrow the car for one day and she's been gone a week--give an inch! This expression, in slightly different form, was already a proverb in John Heywood's 1546 collection, "Give him an inch and he'll take an ell," and is so well known it is often shortened (as in the example). The use of mile dates from about 1900. (Proverb) Be generous to someone and the person will demand even more. (Describes someone who will take advantage of you if you are even a little kind to him or her.) "If you let Mark borrow your tools for this weekend, he'll wind up keeping them for years. Give him an inch and he'll take a mile."
"The grass is always greener (on the other side of the fence)". The meaning should be obvious, i.e. once you've got/achieved something, you'll likely start to want something else.
Apples grow faster from saplings than seeds
It is a much less negative idiom from the ones already mentioned, though maybe a bit... countryside.
In the UK (and probably not elsewhere) there is a set of constructs built around "When I was young ...". Straightforward (non-jokey) usage might be "When I was young, I'd have put on a pullover rather than turning the central heating up". In this form there's a mild element of criticism of the younger person who has never known the hardship of how things used to be, but also a mild element of self-deprecating humour and recognition that the youngster is not going to fetch a pullover and turn the heating back down.
It goes from there through to a fake Yorkshire accent "When I were a lad, I had to break the ice in the toilet before I could have a morning pee" (which was true within my living memory), up to the full "Four Yorkshiremen" TV sketch with ever more hyperbolic and absurd claims, ending with the punchline "And if you try to tell that to the youth of today, they won't believe you!" https://en.wikipedia.org/wiki/Four_Yorkshiremen_sketch
Here's the poetic way of saying it:
“If the stars should appear one night in a thousand years, how would men believe and adore; and preserve for many generations the remembrance of the city of God which had been shown! But every night come out these envoys of beauty, and light the universe with their admonishing smile.”
~Ralph Waldo Emerson