Dominance and submission are not unique to BDSM, it is deeply wired in our psychology as a species. Hierarchy is a natural human phenomenon. We are status-oriented, competitive, and we seek to feel important than and superior to others. No matter how many times we uphold the myth of "freedom and equality", we live in a world of social stratification, rich and poor, ruler and ruled whereby the majority submit to the will of the minority. Hierarchical structures exist in almost every organization -- military, government, corporations, religion, sports teams, families, etc. This alpha-omega model is necessary for us to live cooperatively (Thomas Hobbes asserted that it helps to promote internal order within communities), to make decisions so that it can be carried out efficiently, to ensure the survival of the group for the next generation. This primordial ranking behavior has defined us since the dawn of our race. The ubiquity of dominance and submission in our world is often seen in the form of competition. Charles Darwin noted that we are competitive over food, and, more importantly, over sex. One of the purposes of establishing hierarchy is so that those who land at the top of the pecking order will gain the sexual favors of the opposite sex, typically between males for access to females. Coming out ahead is a display of one's genetic endowments, AKA ideal sperm donor. Yesterday's muscles and hunting skills are replaced by today's cars and bank accounts. People need to know where they stand and who has the power, who is the best, who has the most resources. In other words, who is superior and who is inferior, who gets to mate (and live), and whose genes must die with the generation.
In BDSM, Dominance and submission eroticize an existing element of human nature. The capacity to exert control and dominance over others is just as natural as submission and obedience. The fetishistic slave does not learn how to be submissive, he discovers it and very often from an early stage in life. It comes naturally because submission is already 'built-in' and the naturally submissive will instinctively respond to confidence and assertiveness, mix it with the powerfully arousing image of the female form and the male slave is rendered helpless. Just as sex is necessary for procreation, dominance and submission are necessary for group survival, so good old mother nature made those instincts pleasurable for us.
The relationship between a Mistress and Her slave is a micro representation of power and control. Power, as we all know, comes with privileges. The powerful and wealthy in any given society have more freedom to exert influence, to be respected and heard, to be able to get what they want, and sometimes to be able to get away with committing crimes. The powerful can and will do whatever they please while those of lower status are more limited in their choices and oftentimes must adhere to the decisions of others. The biggest difference between this image of inequality and that of a Mistress/slave relationship is that the latter takes place with everyone's full and enthusiastic consent. The Mistress gets to do whatever She wants as it is Her prerogative by virtue of status and power, the slave must suffer and endure whatever decision She makes even at his own expense -- and both parties would not have it any other way. This is why BDSM is about power exchange, rather than the transfer or seizure of power. It is based on mutual understanding and acceptance of one another.
That BDSM is still labeled as "kinky" and "alternative" is outdated. Look past the costumes, toys, and torture contraptions and you will see that what lies at the heart of it is the art of control and surrender, innate human behaviors. The elegance of BDSM is that it is able to explore and manipulate aspects of our natural wiring, like reshaping and rearranging the toys in our mental nursery. By doing so it thrusts our minds and bodies into a heightened state of excitement and arousal that is simultaneously visceral, sexual, and cerebral. The predilection for dominance and submission exists in all of us, we are all naturally kinky, some of us are just more aware of it.