Why do guys brag? And why would an ex do it?

Ugh I've ran into my ex recently and prior he's been texting me, but he hasn't been flirty or direct, so i don't think he wants back together, but he has been really nice. Thing is, I want nothing to do with him, I've moved on and he knows it but the last 2 times I've seen him by accident, he's bragged. The first was bragging about his job and the 2nd, I asked him a simple question and he answered and went on and on and on about himself....he was never like that, well, he was, but i can tell its obvious. why is he bragging if i don't care? what is the purpose? oh and he left me.