she's seeking the affection her parents were never there to give her, and any signs that someone will give her that affection makes her happy which is why she had so many bfs. She's terrified that everyone she loves will abandon her like her parents did. She acts like a child with the tantrums because she never got the chance to grow up mentally (because of the lack of affection), she treats everyone she's close to with the affection she desires and when they don't give her the same affection she takes it as a sign that they don't actually like her since she's so scared of losing them which causes her to spiral out of control and break off the relationship.
She didnt use to know how relationships work but has slowly started to ever since rihito showed up but by this point she seems to be self aware (atleast subconsciously) of why people avoid her