When a person has been abused they are naturally skeptical and find it hard to trust anyone, so often in a romantic relationship, they are continually 'testing' the others love, and this is done in many ways, some very subtle and others more aggressively. Now if you have been betrayed in some way by your lover, that only escalates the insecurity and then it is hard to believe almost anything they say and you can seek affirmation by continually pushing them away to see if they care enough to fight to get your affections back, and that, of course, can be exhausting for them and you, and of course, you will never be convinced or satisfied.
Now, complicate that with the fact that as teens, we are still very immature in many ways and it is never God's plan to have teens involved in intense adult relationships, we just are not prepared emotionally yet, so we may do adult things (sexual activity) but still act like children.