I'm actually a mountain biker myself, but I enjoy road cycling for similar reasons. In fact, most of my daily biking trips are about 50% road biking and 50% mountain/off-road biking.
I like mountain biking because of the variation in terrain, as well as being able to spend time outdoors and among the trees. I like road biking because of the lack of variation in terrain, which makes for a more relaxing joyride. I live in a rural area though, so even my road biking involves a degree of nature, as my town is surrounded by open fields and valleys.
When I have a smooth road to just pedal down, I can relax my mind and take in the scenery. Mountain biking involves way more concentration and strength, which can be fun but certainly not relaxing, at least not in the moment.