My experience with Salsa dancing outside of our university ballroom club is pretty limited, but I see something similar in the swing community. It goes something like this:
You dance most who you're most comfortable with, and you're most comfortable with the people you dance the most with...it's a cruel circle. Kind of a tangent to this is that people seem to be more comfortable with people that their friends dance with, so there's almost this sine curve of comfortableness that starts with yourself and then degrades away as each person is removed by another layer of seperation.
I.E. I'm very comfortable dancing with FollowX
I'm kind of comfortable dancing with FollowY, because she dances with FollowX's boyfriend.
I'm not comfortable with FollowZ because none of my friends have ever danced with her.
There's something of a heriarchy that builds off this phenomenon, that I've noticed. That is, if a stranger walks up to FollowX just as I walk up to her (remember she's someone I'm comfortable with) the stranger will often defer away, or be more hesitant in asking her to dance.
The psych major in me to my apparent self-confidence vs the anxiety of a stranger asking FollowX to dance. He's anxious, I'm not, so he instinctively interprets that as a rank issue.
I'm not sure if it works like that for girls. They seem to be less prone to "Alpha-male" fiascos.
The kind of interesting thing that I draw from this is that the really good dancers may be as hesitant to ask new people to dance because it's outside their comfort zone as much, if not more so, than because they're "too good" or afraid of being "bored."