Can somebody please explain this guy to me?

So I know this guy and I've known him for like 4 months. And he's a good friend but he's just so weird, I don't understand him at all. Okay so first he's a drug addict, but not like cocaine or meth or anything bad, he abuses his prescription pain killers and he drinks a lot of alcohol to "numb" himself. He's very good looking and funny and outgoing but doesn't chase any girls. He's not gay though, he doesn't chase guy's either. Everyone asks him why he doesn't have a girlfriend and he's like "no one likes me and I drink to forget how alone I am" jokingly, because he knows that a lot of girls like him and he just doesn't give them a chance. He's really friendly with me and he's super nice despite being really popular and I thought he liked me at one point but he never made a move. Like yesterday we were at a party at he was just drinking on the couch and called me over and introduced me to all his friends and put his arm over me but didn't kiss me try anything further. And he's not shy if he wanted to kiss me he would have. I just don't get him, it's like he's always smiling and happy and so nice, like unbelievably nice and wants nothing in return from anyone but he still abuses drugs and drinks like an alcoholic like he's actually unhappy. Why is he like that?