Why men move on so fast after break up?

Recently my bf just broke up with me because he isn't moving back to the United States. It was for 7 months and long distant relationship. Abut a month right before we broke up, I've seen pictures on facebook of him and this girl from work. I asked about him about it and he says they were just friends. then we broke up about a month after that. now the week after we broke up he started to date the girl from work. I suspected cheating, but he says he didn't but dated her a week after we brok up. I'm so hurt and shocked by this and its really hard for me to move on. How can he just move on. Did our relationship mean anything to him?