many reasons actually.

The ones that are his fault: he was insecure, manipulated, she lied to him telling him i was cheating the whole time, she told him i didnt like him, when i made mistakes shed tell me not to tell him then go tell him herself so he put trust in her, he didnt communicate with me when there was a problem, he was constantly being told to leave me by her and his own friends because they didnt like who he became around the time we got together, and he wasnt honest about his own feelings…he wouldnt let how much he really liked me surface due to his insecurities. He wanted to keep his image of being a player too bad to let himself fall first. He wasnt willing to work with me through my depression stage he instead went behind my back and vented to her about how i was acting instead of helping me. But….mostly…i pushed away because out of nowhere he stopped making me feel special…and that scared me the most.

The ones that were my fault: I wasnt expressive enough…i had to high of a guard up. I was insecure…i was depressed and it rubbed off on him…i didnt show him affection enough because of the gut feeling i got about something…And i waited too long to let him know how much he meant to me. I gave him no self worth…i didnt show him i wanted to be there…when we first met i was happy and fun and completely honest with myself and him about how i felt…then when me and her started having my downfall and they started to get closer i started closing off…i started getting sadder and sadder that my best friend was treating me so wrong….so even if we had alone time…i was no fun…i was quite the downer actually.

so basically putting all that together makes your answer. I was never honest about how i felt to him because i was scared and it made him put his wall up as well….we pushed each other away because we wanted to be more in control which let her swoop right in and feed him every lie she told…him being insecure made him believe her and the more we pushed each other away…the more he believed it…and then shed get EVEN CLOSER.If me and him could have set our pride aside and just communicated with each other it wouldnt have happened. The only thing that kept us together even through all that was the rare moments we did have alone where we really clicked….that really just reminded us why we were together in the first place…in those moments nothing else mattered…our walls were down and all the rumors were out the window…those moments alone reset our feelings for each other and got all the bad ones out.

To sum it up: insecurities and lack of communication.