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What to make of this person

Basically I've just had an arguement with the person I was seeing. Basically, he's just spent the past 3 weeks completely ignoring me and upon finally managing to get in touch with me, his excuse was that he did it because "he can." He also explained that I'm not the only person he's done it to and that a lot of his friends are getting annoyed at him because he ignores them and treats them so badly. So I know it's not specific to me.

Additionally, this person is only social when he can go out and get either drunk or high. He's admitted that when that isn't happening, he's happy to just go to work and play X Box by himself and just be alone. Quite a lot of the time, he states that "people piss him off" and he "just wants to be left alone." He's 22, has no qualifications, still lives at home and only has a part time job, so he doesn't exactly have much going for him.

But when I'm actually with him, his attitude is completely different. Last time I was with him, he seemed genuinly happy to see me and was lovely to me, so basically a complete opposite to what I've described above.

I don't understand what the problem is. Does he seem depressed or something? I just can't think of anything that would account for his rapid changes in attitude to not only me, but his friends too.