Here's the real fact boys/Men..... Ladies are really emotional by nature, they easily get overwhelmed. They tend to easily overreact and their mood changes real quick.
By simply liking another girl's picture would hurt her or turning around to look at a girl at a glance would make her ask lots of questions in her mind. It is not immaturity when she does that, it is just that she cannot hide how she feels. She tends to be paranoid sometimes. 
When you are together, it hurts her when you look at other girls. She would think that there is something wrong with her, I know you consider that as too petty,but she's sorry if she tends to overthink.
She knows that she is way too replaceable, she got insecurities, she gets jealous easily. You wonder why she has trust issues ,it's because she's been hurt a whole lot of times.
She just hopes that you are different from the others. And please don't give her reasons to doubt you.
Goodmorning steemians