Why is mixed-terrain night riding so good? The list is long and probably personal. Here are some of the reasons we search for the night rides:

You have all the trails to yourself. No hikers, no dogs, no horses. It's you and the trail. Probably a lot of bunnies. And maybe a co-rider or two.
The trails are quiet and meditative. Everything feels more peaceful at night. Everything is calmer. You can hear your breath. It's unlike any day ride.
The trails you've ridden 100 times feel completely new. The stream next to the trail speaks a lot louder; that drop-off feels a lot steeper; the slalom through the trees feels a lot tighter.
Focus. While life is quieter, there's a lot more need for rider focus. Depth of field, shadow play, and tunnel vision all require heightened attention compared to a sunshine ride. It's a good kind of focus.
You have the roads to yourself. Not only do you get the trails to yourself, the paved roads are deserted. Rarely will you see a motorized vehicle.
Cooler temperatures. In New England, in the summer, the temperature can drop by 10-15 degrees when the sun goes down. That's almost the perfect temperature for any ride.
You are highly visible. Motorized vehicles will see you really easily and give you wider berth that daytime riding.
No blind intersections. You can see cars before you might hear them; headlights are hard to miss.
You can easily keep track of your co-riders. You're unlikely to lose anyone because visibility is so high.
Bonding time with your co-riders. We rarely ride the night solo. It's always wise to have a partner or two; just like on any offroad ride -- the world is not predictable and that's part of the point. We bond even without speaking. There's something about riding trails, light behind light behind light, that brings riders together.
Fewer insects. It's bedtime for most mammals so the flies aren't looking for food. Wondrous.

We could go on and on. Why do you ride the night?