Whats his deal?

this guy i know , i work with , i hang out outside of work with sometimes, ended his engagement b/c his fiancee didnt trust him, he wanted her back but she didnt take him back, this happened about a month ago, he told one of my friends he still missed her and he would talk about when they get back together he would prove to her he had been faithful, so it boggled me when he started to date this girl at work , they hooked up one sat. night and now are gf/bf, but how can he get over his ex that quickly? he was engaged to her and he even told a friend of mine he wasn't ready to date.. so what gives?