What would cause a co-worker you've known for years to leave (for a new job) and say no goodbyes?

He/ she probably was tired of everyone and didn't want to deal even with goodbyes. I've done it too lol. I felt some of my coworkers were taking advantage of me, so left without saying goodbye, never wanting to see their faces again for another minute. I didn't say goodbye even to my manager..cause she was very nice and would ask me to stay and I.didnot want to be in a situation where I couldn't say no ( feel kinda pressured), she called me next morning pissed asking what happened....I kinda miss her but never regretted leaving cause somehow I wound up doing everyone else job for a long time abd it was taking toll on me