When Hermione accuses him of missing Ron, Harry denies it, but admits to himself that while he "... liked Hermione very much, she just wasn't the same as Ron. Even though he does respond to Cho, their relationship ultimately fails for a number of reasons, including grief and guilt over Cedric's death. The emotional dynamic between Harry and Cho is far too complicated. She has sworn to remain at his side during his final quest to rid the world of Voldemort. She even attempted to explain the mysterious ways of girls to him when he was fifteen. Sex is, in fact, the characteristic that defines the difference between a platonic relationship and a romantic one. The lack of fun is one of the main causes of the break down of Harry and Cho's romance. Learning that the relationship has been abandoned, Ron responds, "You're well out of it, mate... I mean, she's quite good-looking and all that, buy you want someone a bit more cheerful." Cho is still grieving for Cedric, and she wanted someone with whom she could commiserate.
Soon after these events, Rowling specifically makes the connection between Harry's humour and romance during his relationship with Cho. Rowling makes the difference very clear when she describes Harry's reaction to other girls. When Harry meets Cho Chang, the moment is marked by his attraction to her. Harry acknowledges his desire for humour as one of his most basic needs after the Tri-Wizard Tournament.
Facing the return of Voldemort, on the verge of war, he gives his winnings to the twins to use as start up for their joke shop.
After kissing Cho, and faced with the prospect of asking her out, Harry considers, "Yes, he had liked Cho for ages, but whenever he had imagined a scene involving the two of them it had always featured a Cho who was enjoying herself, as opposed to a Cho who was sobbing uncontrollably into his shoulder." It is an understandable expectation - to have a partner who enjoys herself in his presence, but the sentiment gets to the heart of Harry's biggest disappointment in the matter.