Why Does He Keep Crawling Back To Her

I would just like to know what people think about this> I dont ever want anything more than a friendship with this person, so its not a jealousy thing or anything of the sort. im just curious to know why??

My friend has been dating this girl for over a year now. The first 5 months were good between them, then jealousy kicked in, then a lot of useless fighting constantly!! she had a boyfriend when her and my friend hooked up, and had sex(2nd night) This girl has cheated on every1 of her bfs. She broke up with my friend,(but they were still together) and she had sex with another guy. My friend refuses to believe any1 that she would ever do that. (Even though she cheated on a boyfriend with him)! She broke up again with my friend,(about the 6th time now)and found a new boyfriend. Had a sexual relationship with him, but was still having sex with my friend on the side. Im good friends with her new boyfriend. He refused to believe she would ever cheat on him because she tells him she loves him! I told him she tells every guy she loves them. Well he went away for a week, and my friend was over at her house, and she would go to his house everyday. Her boyfriend called me and I told him and he got mad at me and called me a bitch. When he got back she broke up with her boyfriend, then told him she was cheating on him the whole time they were together.(2 months) He has apologized to me for everything. Now my friend and her act like they are going out. She kept breaking up with him because hes an ass to her. (how do you respect a girl like that anyway)?? My friend keeps taking her back even though she runs off every now and again for some new d*ck! My friend told her ex that the sex is great for about 3 months, then she starts getting insecure again!!(basically she looks for something new to screw) So whats wrong with him that he has to keep sucking up and crawling back to a girl that cant be faithful to any guy. Too me its the sex, because if it wasnt> instead of him saying that the sex is great for about 3 months, he would have said “Our relationship is great for about 3 months”

Tell me what every thinks. Thanks!!