The Doctor did not love River Song. He felt sorry for her. Their relationship wasn’t a romance, it was a tragedy. If you thought of River Song as the love of The Doctor’s life, here is an argument to give you pause.
Think about it from The Doctor’s perspective. He meets River Song. He doesn’t know her, but she is madly in love with him. She then sacrifices her life to save his. The Doctor is burdened with the guilt of River dying for him. However, it’s not the first time someone The Doctor barely knows has sacrificed their life for him.
He moves on.
Then he meets River again. Still clearly in love with him – and know he knows she will die to save his life out of love. He can hardly keep her at arm’s length, doing so would alienate her and create a paradox. So he humors the flirtation.
Some part of The Doctor must feel guilty that he is leading River on so she will sacrifice her life for his in the future.
Then The Doctor discovers River is actually the daughter of someone he cares very much about: Amy Pond. More guilt. Note that The Doctor never tells Amy that River will die for him.
Then she sacrifices all of her remaining regenerations to save his life, again.
Twice this woman, the child of a woman he loves, has sacrificed herself for him. And The Doctor NEEDS River to be in love with him to preserve the timeline and keep him alive.
Those who will argue that The Doctor does love River will point to The Doctor staying with River at Darillium. However, remember that their stay at Darillium was foretold by River. It was a fixed point in time. The Doctor was forced to be there (regardless of his wishes).
I’m not saying The Doctor didn’t have affection for River; she’d saved his life, was the daughter of his friend and good company. I am saying that The Doctor’s marriage to River was one of obligation and sympathy.
The sad thing is that River understood that The Doctor did not truly love her. She says as much in 'The Husbands of River Song': "When you love The Doctor, it's like loving the stars. You don't expect the sunset to admire you back". She knew.
A story about a marriage of obligation is nothing new, their story just has more timey-wimey in it than most.