Why would a guy kiss a girl just to defend her?

well i saw this in a movie but this girl did something embarrasing (nothing WRONG) and all the popular people r making fun of her so the guy is most popular and hottest in college and evry gal wants him...but he keeps on defending the girl....people would jus keep makin fun of her...and one day the guy is telling the mean gal to knock it off nd she asks him why and so he jus goes up to the good gal and kisses her in front of the whole college...plus he doesnt realize yet that he liked her.... What r sme reasons a guy wuld do dat??!!??