The Doctor doesn’t quite understand what keeps drawing him back to earth. He’s tried many times to figure it out, telling himself it’s because humans have so much potential, which is true enough, and that they bear a striking physical resemblance to Gallifreyans despite only having a single heart, but surely there’s more to it than that.
Earth is a world of many nations, most of which seem to regard all others as alien, regardless of the fact that they’re all the same species. What will it take to unite them as one? Perhaps that’s what he hopes to discover.