Castle has always had luck with the ladies, did Kate expect him to just wait around while she was with someone else and pretty much doing all she could to make him believe she wasn't interested?
Women really need to learn something important. If you make someone feel they're not wanted, even if you don't mean it, there is a huge chance that person will move on. No matter how much a man cares about you, there is a limit to how much most men will take before becoming frustrated. Men HATE heartache far more then women do, and we will give up if we feel like that ache is no longer worth it.
Kate is a very strong minded, independent women but she has always made Castle feel unwelcome, and when she got a boyfriend he felt like he was less important to her, and she did absolutely nothing to assure him otherwise.
So I can't really feel sorry for Kate. She kept stalling and this was the result.