Problems

Yesterday i was talking to a friend of mine, she was really depressed over a relationship that she had been into for the past 5 years. Apparently the person she is involved with does not trust her. The reason being that she being a very pretty lady naturally attracts guys, though that being no fault of her own, bugs the person she is committed towards. Why is it that when we get into a relationship we become so possessive that we forget to take into account the feelings of the person that we are with and say things to him/her which would cause hurt? And also why is it that we cant seem to break away from those people who cause us hurt and pain? What is this feeling that causes us to endure so much? And yet the smallest thing will tear two people apart. Food for thought, is it just the fact that people are naturally selfish and are concerned only about their own self?