Just go & punch up a question directly on his/her face- "Hey whats the matter with you."
You never know what would be the reason behind that. But if u both are gud friends,he/she wouldn't bring u down. There will definitely be reason.
Just let it be, well you cannot force someone to be only with you. But it little too much, how came stop talking with you if the only reason because another person except you make a big mistake to your best friend
