Well, it depends on their personality. They have to be respectful, understanding, polite...and all that. But I can't expect perfection. I know, I know.
If they're a loner, there must be some sort of reason that they have little friends. If I see some boy sitting alone, I instinctively question to myself, "What's wrong with him? Why doesn't he have any friends?"
But alot of popular guys have tons of friends and stuff, and tons of girls that adore them. So I'd never be sure if I was the only one he was in love with, or dating.
I prefer boys that are normal..not too popular, nor too...lonely?
Hope I helped. :]