I have a bike riding friend, to protect his anonymity we’ll call him “Rob”. Rob is a really good rider and a heck of a nice Guy. He rides road bikes and pushes himself hard. He started racing and moved quickly up the ranks. Whenever I mentioned that he should come out and ride with us off-road, he’d say something like, “No way, that’s too dangerous!” That always seemed ironic to me since road racing (even on a closed course) isn’t really a risk-free activity.
He likely based his assumption of danger on imagining riding road-bike-fast on unstable surfaces while rocks, trees, and squirrels are randomly thrown in front of him like some sort of unwinnable video game. His imagination might have not been too far off except that you don’t, certainly not at first, ride at such a fast pace off road. One of the fun things about riding off-road is that it IS a bit like a video game in that there’s lots to learn, and skills to build in order to move on to the next level, and there are lots of surprises.

Few Surprises
With road riding you can ride for days before something unpredictable happens.
(mostly caused by operators of other vehicles)
Smooth pavement stretching out in front of you for miles, a white line to follow to keep you on track, sweeping turns big enough for an 18 wheeler to negotiate, a yellow line separating oncoming traffic.
It’s easy to be an “experienced” road rider and have VERY LITTLE bike handling skill to show for all the miles ridden.
Road riding experience is more about logging long miles, fast average speeds, saddle sores, hand numbness, aero position adjustments, gear shifting & hill climbing techniques, getting in & out of pedal clips, eating and drinking strategies… and much less about bike handling skill.
Road riding can make you so complacent that when, after hundreds of miles of predictability, your tire finds some loose gravel or a slippery oil patch you fall down instantly. You get so used to the routine that the first surprise situation often leads to a crash. This is because your evasive maneuver reflexes are turned off and packed away so deeply (since they are seldom needed) that they are not available to you in the instant that they are required.
Off-road it’s a-whole-nother story.
With traction always in question you are much more in-tune with your bike, its tires, their tread, and the ever-changing earth underneath them. When a tire skids (almost constantly) it’s not a moment for panic it’s just another chance to practice riding it out, learning to control it, learning how to not make it worse, and dare I say, ENJOY it!
Off-road it matters much more which brake you use and when. Shifting your weight on the bike for better control & balance for different situations becomes routine. Balancing at slow speed, no speed, and tight turns is a skill that you get to practice every ride. Knowing that the front tire can loose traction easily on loose rocks and slippery roots you start to anticipate where such a slip might cause you to loose control and you start pulling-up, letting the front tire skip over such unreliable objects choosing others to put your traction confidence in. Sometimes you just jump your bike completely over things that would be problematic to ride over. You constantly need to “pick your line” choosing the best place for your tires to go to get you through.
You can get an entire lifetime of road riding surprises (minus all of the automotive interactions) in just one lap of the local single-track. Speaking of “single-track”, you also need to pay attention, and be prepared to yield to, oncoming traffic.
Surprisingly, though squirrels can be an annoying hazard on the road with their baffling back and forth attempt at evasive out-maneuvering you, I have never had such an experience off-road. I think in the woods they are at home and confident and have plenty of familiar props to use, things to climb or hide behind. In the middle of a slab of pavement they feel naked and fumble around badly in the stark unfamiliar environment.
Make a list of all the reasons you’ve heard road riders say was the cause of their on-pavement mishap. Then consider how silly they sound from an off road riding skill-set perspective:
“I hit a patch of gravel and skidded” (Sounds like fun, why not ride it out?)
“I hit a pothole.” (Why didn’t you jump it, or steer around it?)
“I hit my front brake and went over the handlebars.” (Have you not used you brakes before? Don’t you know that you always first shift your weight back, and pay attention to traction? If your back tire starts to skid that means you need to let off the front brake before you flip? Why would you grab just the front brake?)
“I was run off the road” (Then why not ride off-road for awhile?)
“I was going too slow and fell over.” (Going too slow happens at least twice each ride, maybe you need to embrace it, practice it, get good at it rather than trying to pretend its not part of riding.)
“I was hit by a car” (How’d the car get through the trees to get to you?)

Wonderland of surprise
Rugged terrain bicycling (RTB aka MTB), is certainly not without its risks. You will most likely fall, and if you really push yourself, you might get in a good fall or two every ride at first.
But here’s the return on investment:
First; Bone density. Stressing your skeletal structure, from say… falling, is good for it (in moderation) as the repair from the stress increases bone density. A lifetime of avoiding falls makes for fragile bones.
Second; Bike handling skill as detailed above. Even if road riding remains your primary activity, learning the skills of RTB-ing will make you a MUCH better rider.
So how does Rob fit in to this risk/reward theme? Well… he kinda broke his neck in a road racing incident when another rider, unpredictably, cut in front of him pushing his front wheel sideways and steering him, violently, into the pavement… at high speed… (not anything RTB-ing would have prepared him for).
Now with his neck vertebrae density increased he’s starting to think that maybe off road riding isn’t so dangerous after-all…