If you are already in a new relationship or if you are the one who dumped him. He'll feel like he's not good enough or he's worried that you'll find someone better. Even though my ex broke up with me he was still really bitter about any guy I could possibly have a relationship with.
Sometimes this bitterness serves to mask the guy's sense of loss and vulnerability. Manly men like to huff and puff, make a show of their anger and announce to the world that some woman done them wrong. This is preferable to examining feelings and tending to emotional wounds that invariably follow a break up. Grumpy young men even cry sometimes, but not in public. If your ex loved you--you know, in his own inadequate way--then you shouldn't be surprised by a public display of indignation for all the hurt he's struggling to process. The bitter approach can be very demanding as a break up tactic for it requires vigorous, almost obsessive amounts of self torture, and we see it most often in young guys who haven't yet learned positive ways to move on.