Some become involved in BDSM out of a need, desire, or love. The need to submit is very strong in some people, and with the right partner, that need can often override their other basic needs. For some it becomes the strongest motivation in all of their life choices. In this case the need is part of their psychological make up and often shows itself whether they are in a power exchange relationship or not, through such things as doing things for others specifically to make that person smile. In the case of a submissive, I think that is a need as well. Often I hear other submissives saying that they did not have a choice, and I can see how this might be true. For allot of people, they have a desire to be useful, to do as others say, to do what has to be done, without any real desire for "payment" it's simply their natural psychological state of being. Some do not have that desire, but experience a need to please in other shapes or forms or in certain, situations.