Yeah, it's not just a question of when he felt something for Jack, it's when he admitted it to himself, and then let himself show it. You could say it all crept up on him. I'd be tempted to say he was painfully aware that he was forming an emotional attachment but chose to push it to the back of his mind. Jack's almost permanent death forced him to confront it.

I guess he was distant in KKBB, but whether that was "you've been away and now your hot ex-lover is here" or that he'd never let Jack know how he felt is pretty open to interpretation. That kiss in To the Last Man certainly took Jack by surprise, but then like fodian pointed out, feelings are hard; making out is easy.