Falling for my best friend?

ok so, my best friend (who is a guy) has told me before that he has feelings for me. i’ve rejected him a few times because i always thought i didn’t like him in that sort of way. however, he’s been talking to another girl for a little while and for some reason i am a little jealous. i know it’s a selfish thought but i’m so confused as to why i feel like this :(