I think part of the problem is that he never seems really comfortable having a mic shoved in his face after a match. I've seen him on TV in much more relaxed situations (like A League of Their Own) and he comes across as a genuinely nice, funny bloke.
I'm so happy that he won though. I honestly think that in virtually any other era of tennis, he'd be close to double figures for Slam wins by now. For him to have won as many titles as he has is quite remarkable when he's up against possibly the best ever player in Federer, the best ever clay court player in Nadal, and someone like Djokovic who could easily overtake Federer and be known as the best ever. All 3 of those will go down as 3 of the best 5 tennis players in history, and Murray has pushed them, he's matched them, and on occasion he's beaten them. I don't think any of us would be surprised if he went on to defend his Olympic crown either, or go on to win the US Open next month. Hell, I wouldn't be too surprised if he did both (though I think the heat in Brazil may be too much for him).