That is a million dollar question.
I can only answer what it was with me - he hurt me so many times, that something inside of me simply snapped. They say that the definition of madness is doing the same thing over and over again and expecting the same results - I honestly don't know the count of how many times I went back to him, simply because I loved him, because I couldn't live without him, because I knew he loved me too and because I foolishly thought that that "this time" would magically be enough, over and over again.
The best I can put it, being in love isn't enough, you have to feel loved. I didn't feel loved, and obviously he didn't either. There was an emptiness inside of him that he was trying to fill - by sleeping with as many women as he could, giving his all to work and traveling constantly, not standing up to his mother when she regularly spoke rudely against me - though the times we would spend together were wonderful, I realized that I will never be enough for him. He survived without me, I am not sure Tristan could.