Back-handed compliment of the decade!

"You'll make someone a great boyfriend one day."

Why do I only hear this from girls who are already accounted for...?

Translation:
"You'll make someone a great boyfriend one day, but not me. And hopefully not any of my friends either."

ouch! But I'm sure not really meant with that meaning in mind!
Quite... What does that say for how you are now?

One day you'll make some one a great boyfriend.  Just not now...
I've always found that there's a real danger of reading too much into these sorts of comments, for example the near infamous

"It's not you, it's me"

Can be taken as, "You don't look like [Brat Pitt/George Clooney/Tim Hughes], and I can't deal with that".

You could probably spent years of your life agonising over that sort of thing and the only outcome is that it'll move you to being bitter.

Best just take it this way, Girls in a relationship probably feel more secure complementing guys because it's less risky of being taken the wrong way, fear of things being taken the wrong way is a whole host of social and rejection fears that everyone has. Don't worry about it, take it as a complement and don't give up.
Oof, hadn't spotted the time element to the comment - thanks for pointing that out Stoney!

But yes, Dan and Jen, I knew what my friend was trying to say, I'm just over-exaggerating the negative connotations...

Like the Murphy's, I'm not bitter.
"You'll make someone a great boyfriend one day, but not me. And hopefully not any of my friends either."

Ha ha, that's so true.  Girls love giving compliments on how they can't understand how you/your friends are single until you try to implicate them or their friends to back up their comments with expressions of interest.