Why do some guys like girls butts?

okay so im 13 and on the last day of school, we where allowed to where booty shorts (no im not a slut i just wanted to piss my school of bc i got in trouble for wearing a shirt where my bra BARELY showed) and my friends and i were walking and my bf was behind us with his best friend, and my friends saw my bfs best friend elbow him and say booty shorts, and pointed at my ***. and then me and my best friend went to the mall. and then we were wearing shorts, i mean its summer? why not? and these like 18 year olds were looking at our asses? why do most guys like them so much?