Why did he cheat on me time and time again?

I know ive posted this before but it plays on my mind,I met somebody threw a friend and we started seeing each other,hes ex girlfriend was 6 months pregnant at the time but he told me they were apart,i founf texts on his phone from her basicaly like they were still together,but he always said they wernt and it was just for the baby,we had and the first time i fell pregnant even tho we used protection,Then there baby was born and we kinda drifted apart,i decided not to keep the baby and he didnt ever bring up the fact i was pregnant he just kind of ignored,scared or shocked i dont know, a few months later we started texting each other and sending pics and texts,i found out that he slept with a girl called mandy and i was upset and the mother of his baby found out about her and get upset too,We started meeting up again and then he would dissapear for months ,3 years later with this still going on,we started meeting up again but then i hear he has another baby by this girl that he deneys is hes but later i find out it his hes,anyway we started meeting up again and i was shocked to find out he had a girlfriend where he lived for a year,not the mother of his kids another woman,she found out hed been seeing me and ended it with him.Then we officaialy got together last june until jan this year ,i never went to his house tho,or met his friends,we went to the pics once but other than that we just drove around and had in his car or over the bonnett,i loved him tho untill new years eve where i come across on facebook that his back with his ex girlfriend who he has kids with.He told me he loved me and wanted me but i dont understand what went wrong,i still love him but i confronted him and i havent heard from him since,why did he do this to me again?