Why are guys such jerks?

I'm 13, this guy named Guillermo used me to get another girl Jasmine (who is as ugly as hell) jealous. He's stupid also because hey was flirting with her right in front of my face so now i'm not talking to him at all. Then theres this site called "Runescape" its like a video game and i had found out that he "married" Jasmine and i asked him why and he said that he was bored i didnt gat mad because it was so stupid and funny.