What do u do when ur Bestfriend.... ?

Ok first of all, I neither have a girlfriend nor a boyfriend so my bestfriend can't steal anything which I don't have :p Ok enough with the jokes.... Coming back to your question... Hmmmm.... If this was to happen with me, I will consider a few things like do I love my bestfriend more than my BF/GF if yes then I won't do anything instead be happy for them. BUT if that's not the case then I would dump my BF/GF because it's clear that I'm not the one for him/her because if I was then he/she would not have left me. I would be pist at my bestfriend ofcourse I'm only human but I don't think that this kinda thing would force me to breakup with her. I mean, 'bestfriends' means 'friends for life' and I'm damn sure about myself that my bestfriend would never do anything to hurt and neither will I. (Pardon my language)