Maybe because he’s done so many selcas over the years that it’s a habit now? I don’t really know the reason, but i doubt he’s becoming narcissic (i’m basing this on his selca history – he would have become that way looong ago, since he loved taking selcas even before debut.Twitter is just more accessible by fans around the world, hence a big number of photos). Also, he’s not confident enough to be narcissic.
If you want to know my reasoning – i actually think that’s a psychological problem. I suspect he’s insecure about himself and that’s why he’s taking so many selcas, to make himself feel better. It’s like he has to prove to himself that’s he is okay, that he’s good looking and all – this is a common problem and many people suffering from it are doing the exact same thing, me included (except i don’t post them, but hey – i’m not a public person).