@RelationshipProblems,
Engineer has it on the money.
The possible motives are:
- the 'best friend' actually likes you, so can't stand seeing you happy with his best friend' or
- the 'best friend' is jealous that his best friend has moved on to a new best friend
Neither of those are fixable by negotiation.
Other possible motives are:
- the 'best friend' is hearing rumours about you that he doesn't like
- some other thing not related to jealousy
These are usually fixable through talking (though not always).