Fwiw I don't think group rides are competitive at all. I do the fast ones for a few reasons:

1. Training: I usually have to push hard on group rides. While I can also push hard on my own, groups force you to push in paces where maybe you would let go otherwise. They also help me improve my pack riding skills which I obviously cannot do on my own.

2. Speed: Riding in a group is considerably faster than riding alone and riding fast is fun.

3. Measuring my improvement: This one could be considered competitiveness, but I don't really see it that way. Group rides allow me to see where my fitness and skills are in relation to others. I don't care whether I can beat this or that person, but I do like to be able to tell whether my training is paying off or not.

Now, I know some people just ride for the sake of riding and enjoying the scenery and whatnot but me, I ride to become better at it-I think of it as a sport. Group rides fit right in for me.