HOw can I tell my best friend that I l’amour the guy she don´t likes?

The thing is that i like my neighbor very much, we know each other now almost 10 years and I think I have a crush on him. I told my best friend about that and she laughed at this because she thinks that he´s too childish and a looser :( She laughed at me so I a dit the suivant time that I don´t like him anymore but I still do so I´ve met him last weekend and now im sure that I have a crush on him. He already had a crush on me years il y a but i ignored him and now its the opposide :( What can I do?