This is where I go into my "how come everyone loves David Tennant, including me?" post. I first noticed David in He Knew He Was Right where he played a most unlikeable character. But I liked him, a lot. I watched the whole thing awaiting scenes with his character. Then virtuousdream had an icon of him which I saw, and I was a little curious that after seeing him in that one mini-series I recognised him and remembered his name. Next I read that he was going to be Barty Jr. in GoF and the next Doctor (Who). Then I started seeing "I love David Tennant" posts in random places, by a myriad of people from all around the world and of all different ages. Then I saw Casanova and found myself sitting open-mouthed saying "I love David Tennant, oh no, virtuousdream is going to kill me."
He is wonderful, but so are many other actors and they don't get the internet abuzzing as much as you'd expect. D'you think he's put us all under some kind of spell? His charisma is off the charts. Is it his wildly expressive face? His big gorgeous eyes. His geekiness? His beautiful voice? (Oh, but I've heard at least three different accents and liked them all.) Maybe it's the way he embodies a character so thoroughly whilst still being charming.
I'm not in the habit of boyfriend stealing, so I'll leave it at that. It's just... David Tennant = Fangirling.