It's to flaunt his so-called victory of "getting the girl". It's rather immature that he's trying to get the other person jealous. Some may even argue that it's rather low that the guy is essentially treating the girl as a prize of some sort to win.
Why would a guy do this? He likes a girl and he sees that her friend likes her too. So he ask her out, she says yes and he rubs it in the other guy's face
It's obvious so he can get the other guy jealous .