I have obsessed over this alot - watching who people choose to dance with. Let's start with leaders. Many women accuse men of choosing youth and beauty, or the possibility of action off the floor . Followers will also claim they are too good, and men are scared of them, or alternatively, accuse leaders of only asking the best. Followers also look at the success of the aggressive women - who don't wait to be asked, but stand in front of guys and ask them directly some other way. (I have a student who hides in the bathroom at milongas to avoid some of these women. I've developed other tactics). My student got royally yelled at by a practice partner because he spent too much time at a practica with a different practice partner. That caused him to dance more with her at the milonga, although I suspect it could have turned out just the opposite. I tend to think it's comfort-you ask the person you know will be accepting and nice to you. I also counsel followers to smile, and be where guys are just coming off the floor. Wearing a bright color helps, too. So, what it the REAL reason you ask one person over the other? C'mon, guys, spill!