I mean the movie made the comment that he was part human on his mothers side or something but it has been brushed over and sort of ignored.
It is probably a number of things but the main one is going to be that people are making the show and it is easier and cheaper to use real locations on Earth and have humans figure in.
In story he finds them fascinated by them. Sort of humans in general, their drive, ambition, humor and so on. This has varied a bit over the course of the show I think but he likes that it is hard to predict what they are going to do but are capable of great and wonderful things. Or terribly stupid things. That last part he is less of a fan.
In a way it is the same core wanderlust. He left the Timelords initially to see the universe and get away or similar reasons. Humans also are explorers by nature.
One would expect the Doctor to have had more alien companions and he has had some, but again an Earth show.
Made perfect sense when he had blood ties to humanity, but now not so much!