Gale guilts her into it. She doesn't have any romantic feelings for Gale, but she knows he loves her that way. So, whenever he is hurt o sad she kisses him to make him feel better. However, you'll notice afterward she wants to pretend like it never happened. Where as with Peeta there are multiple times she gets annoyed if he doesn't baciare her.
Well, i think she's kinda had a crush on Gale since they met, but in the Hunger Games, her and Peeta had to pull the Amore act, and at first it wasn't true, then slowly she began to really Amore Peeta, but at the same time she also likes Gale, so she just never makes up her mind (Well technically she does at the end of MockingJay)