The calm.
The relaxed.
The scenery.
The fact that I an do "it" (and most can't).
I will admit to one side-benefit: If I am in the mood and get dropped by my usual non-rando group, I can catch them, and late in the ride dish out the punishment, saying to myself "this is only a 40, 50, 60, 70, 80 mile ride; no matter how bad you feel, now matter much you trash your legs, you KNOW you can make it back."