Humans have it pretty weird, especially when it comes to emotions. So, you find someone, fall in love, promise to never leave them, a few years down the line you realize you cannot do it, and you decide to go your own way. And this happens as the last resort in most cases, when things are beyond repair.
Still when the other person moves on, it will bother you. Even if you left them in the first place. Even if you have already moved on. Why? This is the same person you wanted out of your life, then why does it matter now? This is the same person who probably pleaded you to not leave.
Serves you right, though! Doesn’t it?