Neteyam
It would be due to an argument you two had and you would call things off due to it and it would cause you both to be deeply hurt.
Lo'ak
It would be due to him thinking he is not good enough for you and would break up with you.
Kiri
It would be due to her drifting away from you which in turn causes you to think she doesn't want to be together anymore and it causes you to break up.
Tsireya
It would be due to her being very protective of you and you feeling she doesn't value you and thinks you can't protect yourself which causes you to break up with her.
Aonung
It usually is due to him liking to start trouble with others and you hate seeing this and it causes an argument which causes you to break up.