He fits doctor really well every doctor has a new personality so shut the **** up, I enjoyed matt smith more than tennant everyone has their doctor in the series eventually leave so stop whining
gimme like ten minutes to find the full page of this on TvTropes
When each doctor regenerated, they changed personality to fix the problems they had before.
10 was waaayyyy to involved in his companions, and couldnt bear seeing them go. But all of them go, eventually. So when he regenerated he became a wacky, off-the-wall alien man so that no one would get too close and wouldnt fall in love with him quite so easily.