The one I had that started crying on the potty was because they had figured out the rest of the gang continued on playing without them so they were treating it like being punished - think time out and being put on a stool.

I just carried on, talked about being a big girl now and that as part of that peeing in diapers was no longer acceptable and talked about the privileges of being a big girl and how getting into underwear would show me she was big enough to do XYZ at my house - ie something I know she would like and the babies weren't allowed to do. If she was playing with a particular toy when it was potty time we started putting that toy into a basket and bringing it to the potty with us where she could see that no one else was touching it while she was gone and that helped a lot.

I get the not forcing them to sit on the potty if they cry but let's be honest they can cry all they want but we still make them go for naps, wear their hats or shoes outside, use utensils to eat meals, etc. so there comes a point where using the potty is just what one does end of story so to speak. I am not easily swayed by tears for anything.