Why would he act this way?

okay so i cheated on my boyfriend, but that's not really the point. i thought at first he didn't know, bcuz i've been dating around with this really hot guy behind his back for a while now and i had with him about a week ago. but then i started feeling like he found out but wasn't really saying anything about it because he acted pretty depressed around me and talked super-quiet, didn't really smile much either. so i got the feeling that he found out. but for the past week exactly RIGHT AFTER i had with this other boy, he got really happy again. i thought maybe he just doubted me at first and then got over it. but no! last night, when i went to his house i asked him if he knew and he said yes, he started crying but at the same he kept trying to change the subject we were at. i yelled at him because he was pissing me off! everytime i asked him why he didn't say anything about knowing that i cheated on him hee changed to subject. and when i yelled at him he started crying more, lowered my voice and said ' why would not tell me?' and he said 'bcuz i didn't want you leave me.' i was desperately trying to put together what i just heard and give him a response, i couldn't get my head around the fact that he knew i'd been cheating on him and he didn't say anything about it. i was so confused i didn't say anything and i just left, and b4 i even drove away from his house i must have sat with my head on the steering for a half hour thinking about why he act this way. what, is he desperate? i don't get it! i am not at all mad at him (i have no right to be) i've just never heard of some1 being cheated on and living with it!