BDSM can certainly be full of tenderness and love, and in fact, it often is. The experience is very profound, and very emotional, much more going on than just the purely physical. What many forget (or doesn't know) is that the two most important things in BDSM is consent and trust. The whips and the chains are just nice add-ons. *g*
I have met quite a few people who switch, from dominant to submissive, as their mood or needs strike them. But it's not very common that a dom and a sub switch with each other only. It all depends on personality though. No one can comfortably be dominant 100% of the time, and a dom has moments when they feel small and vulnerable as well, in my experience. :-)