May sound funny, but it’s not funny. Trust me it happens quite often. One of my friend once had a crush on his best friends girl friend when he was in class 11.But then he was too young, so one can't blame solely for the kid within him. Instead you should blame the hormones if you are planning to blame one. .
Well what happens when your best friend falls in love with your gf/bf? .Nightmare!! no no its not a nightmare. But I would like to say the same thing. Go talk to your best friend. Explain him/her the importance of "your" gf/bf in your life. If he/she is your best friend then they would surely pay heed to your words. And another advice for best friends. Wait and watch method works well at times. If your best friend breaks off with his girlfriend/boyfriend then its time for you. Not at all gross, but this is my idea.