There is often a distinction made in prose between "loving" and "in love with". It doesn't always imply reciprocity, but it almost always implies a difference between platonic and romantic love. A person can "love" their sibling, parent, or platonic friend, simply by being closely emotionally attached. The same is true for those with whom you have a romantic relationship. Being "in love with" a person is to be in a state of romantic infatuation with and connection to that person, which would be inappropriate for said siblings, parents, and platonic friends, but just fine for a significant other or spouse.
When differentiating, such as "I love you, but I'm not in love with you", the speaker is stating that he/she is emotionally attached to the other person, but feels no romantic desire.
So, the rule of thumb is: you can love your neighbor and you can love your spouse, but you should be in love with your spouse and not your neighbor.