Well Castle wanted Kate, she thought the worse of him and him probably her. She was uptight, no fun etc...once Kate gave in and wanted Castle back it was end game. Just like 1x10 more or less foreshadowed Kate solving her mother's murder. If you watched the first episode compared to Always or even later episodes from season 5 on the goal had been for Castle and Beckett to be together.
Season 3 foreshadowed 3x24 by the talks of getting shot. Rick saying how it's different when it's up close (can't remember the case, maybe 3xk), Royce saying how bad it hurts to be shot, 3x23 Montgomery talking of retiring looking back those were all red flags that something big had to happen.