Can't understand him

I have known this guy for 3 years now. When we first met our relationship was a bit more than friendly but in the end nothing happened because apparently he still had some issues with his ex so I backed out cause I refused to put myself in a competition between me and his ex. So we stopped communicating. A few months went by and he called me. He said he broke it off with his ex for good this time. But still I had my reservations and we just kept being friends. We didnt talk very often. So two and a half years went by and we just kept in touch and talked on the phone about once a month. During those two and a half years he had some personal issues and he would ask for my help so I would always help him out. About six months ago I get a phone call from his girlfriend. Not the girlfriend he had when we first met. Truth be told, he never mentioned to me that he had a girlfriend. Anyway she told me how long they have been together and that they got into a fight and she somehow managed to find my phone number and called me cause she wanted to know what exactly was going on between me and him. She asked me quite calmly, she wasnt yelling or anything, that she wanted to meet up and talk. I was very shocked with this call cause I was not expecting it at all. Anyway I accepted to meet. When we met I made it clear that nothing happened between us. What I cant quite understand though is that she knew about me and our friendship, he told her but he never told me about him having a girlfriend. Anyway after the meeting I completely stopped any sort of contact with him cause I didnt want to involve myself into this weird situation. About 2 months after the meeting I accidentally run into him. The minute he saw me he just avoided me, pretended he didn't see me and discreetly left. I am 100% sure he saw me. After a few days had gone by and I kept thinking about it and how he left feeling, I guess, a bit embarrassed by the whole story. So I texted him and told him what is done is done no need to ran whenever he saw me. Needless to say though we didnt really talk much after that. A few days ago it was his birthday and I just texted him to wish him happy birthday. He called me and he told me that he broke up with his girlfriend (apparently they always fight and then patch things up) but he wants her back and he asked for my help. I didnt want to be dragged into this whole mess but I said yes. Maybe I shouldnt have but I was a bit curious to tell you the truth. Anyway we met up, him and me, he told me why they broke up and if you ask me their whole relationship was one big mess. The reason they broke up was both of their fault. It was like 2 immature and selfish people got together. This time their fight was serious, not like other times. This time it would be very hard to get back together. I gave him my advice and how he should behave but deep down I know that this break up is for good. But of course I didnt tell him that. It has been 2 weeks now since their break up, they dont talk to each other and basically ignore each other. He still wants her, he is not exactly having the time of his life, I think. But they both did things to each other they never should have. My question is why did he keep his girlfriend a secret from me but told her about this female friend he has etc etc? And why did he ask me to help him get back together with his girlfriend when we sort of stopped talking to each other? Especially when he hid the fact that he had a girlfriend in the first place? Why did he involve me in this story?? I only see him as a friend nothing more. I did think of him as something more than just a friend for about a year since we first met, but after that I just got over it. Now he is just a friend.