A lot of people. He loves his companions as friends. He had a family on Gallifrey, including his granddaughter Susan who he clearly loved platonically. He was very sad when they stopped traveling together, as he is with most of his companions. But you probably mean romantic love, so I'll cut to the chase.
He had a wife on Gallifrey, so it's very likely he loved her. There may have been some romantic chemistry with his companions in the Classic Series, Jo Grant, Sarah Jane, Romana, ect. But that's entirely debatable.
The New Series is more explicit. The Doctor loved Rose, and was about to tell her on Bad Wolf Bay, but he didn't get to. He cried because of this. Eventually his copy made from the DoctorDonna meta-crisis did tell her.
the Doctor probably loved River Song, since it's implied they've had/will have a physical/sexual relationship, and they've been seen flirting and kissing heavily. Eventually he married her, which was at first implied to be him getting River to kill him to save the universe, but since it was later revealed he told River he would fake his death, him loving her seems to be the only explanation. In "Let's Kill Hitler", he whispers something in Melody Pond's ear to tell River Song, to which she says "I'm sure she knows." when Melody realizes she IS River Song, she gives up all he remaining lives to save him, so he probably whispered that he loves her.