It's obvious why doms want to be doms, but I'm interested in your thoughts about why subs want to be subs. Please share your thoughts...and motives.
To answer this question for myself, I first question why you would assume that it is obvious why a dom would want to be a dom.
I'm a switch, for me this means that depending on the day, the time, the position of the third star to the right of Jupiter, as well as countless other things going on in my life at that point in time causes me to change whether I feel submissive or dominant.
Because of that, I believe that there is not really a reason why a person wants this or that. Sometimes people just want something.
In terms specifically related to the appeal of being a sub, when I'm in the mood, its nice to have someone else take control. For a brief(or long) period of time someone else is directing the action, and is there to catch me if something goes 'wrong'. The dom has to(in my opinion is supposed to) watch out for the sub. When I have a desire to serve, it means that for that time, I exist to fulfill the needs/wants of the other person, and if I 'mess up' or 'fail' the dom will make sure I dont get harmed.
Its the ability to place my trust in someone else, and once the trust is placed, the rest of the world can go away for a while and take the crap with it.
Ans of course there are the times when its hard to have that trust, and things need to be tweaked from both parties and things do go wrong, but when things are right, its a 'freedom' in its own way.
Disclaimer: These are only my opinions from my experience, Sorry that this was incredibly long winded and if it doesn't make sense.