5. How they make you feel is sexy

Research shows we don't really fall in love with a person — we fall in love with how we feel when we're with them.

This is best demonstrated by the concept of emotional contagion: We're bad at telling what made us feel a certain way, but good about making associations.

Feeling excited, stimulated, and aroused is often associated with the people around us, even if they're not the cause.

This can be taken to extremes: having someone try to kill you can actually make you more attracted to them.

Those in the high-fear condition did show, for example, significantly more desire to kiss my confederate (one of the key questions) and wrote more romantic and sexual content into their stories. Looking at the details of these results, I found that the situation had generated, quite specifically, romantic attraction. [The Heart of Social Psychology: A Backstage View of a Passionate Science]

Some might find the notion that we're just "feeling junkies" un-romantic, but this  idea can save relationships and prevent divorce.

By doing things that rekindle those exciting feelings, love can be restored:

…if partners experience excitement from other sources (such as novel and challenging activities) in a shared context, this shared experience can reignite relationship passion by associating the excitement with the relationship. ["Marital Boredom Now Predicts Less Satisfaction 9 Years Later" from Psychological Science, Vol. 20, #5]