Everybody seems to have some pretty friggin high horses. Most relationships have a period when they go from good to bad. They then get to a point where you don't want to be with that person anymore. Gaining interest in someone esle is a pretty common factor, I would say. Just call it the straw that broke the proverbial camel's back.
Its not like he was screwing around until he finally got around to breaking up with the girl he got tired of. Many guys (and many girls for that matter) would have tried to string both along until it blew up in their face.
I wish SmirnoffIsTasty the best with his new girl. Take it slow, there is never a rush.