1. The Doctor Needs To Evolve
No static character can remain interesting forever. That's why old style sitcoms are boring. That's why serial shows like Doctor Who--even when the episodes are all stand alones like last year--focus a lot of attention on the evolution of the characters. The companion who walks into the Tardis can't be the same companion who walks out of the Tardis. The same goes for the Doctor. And falling in love is part of how the character of the Doctor has evolved. The Doctor lost Rose Tyler, twice. And he lost River Song. He lived with Rose knowing he would lose her one day, and every time he met River he knew he was leading her to her death. The things that has done to the Doctor have been fascinating to watch. Knowing that he'll have to go on alone, knowing that if he gives himself totally over to a romance he'll only get his hearts broken. Falling in love anyway, dealing with the consequences. These things changed the Doctor. Maybe for the better, maybe for the worse. But he is changing, he is evolving as a person. And that's definitely for the best.