I fancy this boy and he has a girlfriend who is really nice . what do i do?

Im a boy an this is what most boys would like you to do: When i had the girl who most people thought was the most prettiest girl in school(i don't meen to be big headed)i thought i couldn't get a better girl.Then 1 of my mates came up to me(it was a girl incase you are wondering) and said "i want you to know that i really like you". I just said " thanks for telling me but im happy with my girlfriend". Then,that night i was thinkin about that girl and i was thinkin how funny and kind and sweet she is but she was just like a normall average girl in looks. The next day i said to my girlfriend "i can tgo out with you because im in love with another girl. Now im still going out with that girl right now. So don't think that your looks matter.They really don't its the person inside. Tell him. He deserves to know.You never know he might do the same as me.