Do you remember?

I was thinking about a candidate that I interviewed for one of our job openings, and I was having a hard time pinpointing what it was about the individual that put me off. It dawned on me after a little while that although his technical skills were probably sufficient for the job, he was harboring significant resentment. His ex-wife, his old bosses, his family. Everything in his life has done him wrong in some way. I didn’t have to pry. He was very open and straight forward with his unhappiness, and his overall level of dissatisfaction with EVERYTHING.
It oozed out of him. I would ask a technical question, and his response would start out as a technical response. By the fifth or sixth sentence, he was moving into personal territory, and by the tenth, he was explaining how the answer was someway related to a person who had stolen his promotion, or slept with his ex. At one point, he realized how negative he was sounding, and mentioned his new girlfriend. He explained that she was “good,” but then launched back into descriptions of how he had been wronged by his ex.
I haven’t had the hardest life. It hasn’t been all roses, but by and large, it’s been, at the very least, acceptable. I have things I am proud of, and things I would like a do-over on. That said, I am as entitled to resentment and unhappiness as this guy is. But I can’t imagine living that life. It must be so tiring to have to carry around every slight that has ever befallen you. At what point does a person learn to just let it go, and just be? Feel free to respond to this. I am truly interested in your response.