During the movie, Melvin is very Neurotic.. and has a tendency to either say the wrong thing, or say something that is just off.  To strangers, associates, love interest (girl).. Often, even if he is trying to be nice, he offends people.  Often he does this when he is not trying to be nice, but not because he is mean(even though he kind of is perceived to be) but his neorosis, for whatever reason will sort of prevent him from sharing a warm moment with another human being.

This scene, near the end, where a homosexual who Melvin is 'friends' with ...(Even though Melvin has said somewhat horribly offensive things to in the past, including comments about him being gay, not because he is homophobic necessarily but because he says horribly offensive things to everyone in general)   tells Melvin he loves him (Understanding his neorosis and wouldn't be expecting a warm response anyway)  Melvin... by saying  ''I tell you buddy, I'd be the luckiest guy alive if that did it for me.'' .. he does three things (subliminally, not quite sub consciously)  ..He calls him buddy, acknowledging he is his friend  and  says he would be the luckiest guy alive  if that did it for him.   He would be lucky .. ((if that 'did it for him'  which the first answer on this site explains ))..  because the gay character is a great person.  He loves him also, but can't quite say this... .  And by comparing himself to a hypothetical gay person, without any offensive comments, it shows deep down he has no real predujice. ((I'm not, but if I WAS i'd be lucky... cause I do love you... just not in the romantic way))     But he does love him, and he made great strides in his nerosis but not quite ready to actually say  I love you too man...