Reciprocity

The following does not apply the same way to stalkers and maniacs, of course, who have out-sized expectations and screwed-up emotional radar. But in general, (my husband told me this from one of his many shrink classes);

If you like somebody, they probably like you too.
Maybe not quite as much. Maybe a little more. Maybe they want more, or less, than what you offer.

And then there's the other kinds of reciprocity.
If, when you talk to someone, they make you feel helpless, that is how you learn how helpless they feel themselves.
If someone makes you angry, they are showing you how angry they are.

It explains so much of the emotional landscape. I think knowing this is a way to feel reassured, (of course they like me) or a way to resume control of one's own reaction (that's their anger, not mine), or a way into greater involvement or compassion (this is how helpless they feel; that has to be dealt with first).