I just saw again this video of Sebastian Vettel in Top Gear's "Star in a Reasonably Priced Car".
I really liked one of the comments for it: "Damn, he's annoyingly nice! I REALLY want to hate him but I can't" (by perrotthill). This just sums Vettel up completely - he is a genuinely nice person. I am not sure if this is because he is still a child inside (let's face it: he's been racing ever since Schumacher won his first titles, this does not leave a lot of time to grow up) and he is just happy with whatever he can get. Or maybe this is just character - warm and smiley.
The thing is: it is not hard to be happy when you are doing well (i.e. Messi, Barcelona, ...) but being happy even when you are not fulfilling your potential takes a lot. Hamilton obviously cannot do it. Vettel is still this guy that was dreaming about being in Formula 1. I don't think that even in his widest dreams Seb ever imagined he will be a double world champion before his 25th birthday. Thus he is just happy, even with being second on the podium - as long as the fight is going, Seb will be there with his big warm heart. This is why even Brits love him:)