I still wonder why??

ok my now husband, a year ago while dating/engaged came back from a out of town (worked on road) job which he was gone 3 months. When he got home everything was fine then out of blue he said he didn't wanna be together and I found out he was talking to this girl. He broke up with me hung out with her and had sex with her multiple times. Long story but we got back together he promise me he would never do that again etc etc. Well fast forward to this past summer everything between us was great (so I thought) I mean our relationship was perfect. Then one night I was at work and seen he added her to Facebook again, and he acted like it was just causal, when I got home I blew up an left he ended up meeting her that night and had sex with her. Then tried to lie about it on his way coming home, he said he loved me and he don't know why he done it etc etc. . But looking at his phone he was talking to her for weeks.. We are married now and he promised to God and made a vow that he would never do that to me again. But if he said that the first time what possessed him to do it again? I mean It still hurts me to think why why go back to her.