If one were to ask him why he took an interest in Her of all people, he wouldn't blame them. Most of his friends preferred the other girls, like the beautiful Cordelia, the kind Sumia, the bashful Olivia. What drew his attention to Her is beyond even him.
At first, he spoke to her strictly on an intellectual level. She was skilled in magics he knew nothing about. As a tactician, it only made sense to gather information on the dark magic the Grimleal embraced. Even past that, he found himself fascinated by just how much her curses were capable of.
But even as her research continued, that topic was soon wrung dry, yet he continued to associate himself with her. He stayed close in battle(to observe her closely he told his comrades), invited her to his tent, tried to engineer the chore rotation so that he would share duties with her often. Eventually, he found that her devotion, both to her research, and to him, was strangely charming. Her will could stand up to the likes of Chrom, Sully, or Panne, if she wished it. He thought to himself that she could do anything, if she wanted to.
Including doing things no one thought she ever would. He'd always thought her research took top priority, even over her affections for him. But a single offhanded comment changed his mind. When she tried her best to be "normal", he could see how much he meant to her. It didn't matter then, if the others didn't understand it. He didn't even understand it himself, but he knew that she was the kind of person he wanted by his side.
This is short, even for my usual standards, but I really had fun writing it. I'm sure it's easy to tell who my usual choice of partner for Awakening is now, but I don't care. Tharja demands love.