Because US society places a lot of value on independence and personal freedom, the stereotypical reaction to the concept of submitting to another person is often one of revulsion; because it is a kink, it is almost never talked about casually (and when it is, it is often given labels like 'sick' or 'perverted'): these two things combine to make discovering and dealing with one's submissive tastes a rather difficult and sometimes traumatizing task.
Where do these submissive tendencies come from, and what exactly are they? Like all kinks, submission is a matter of taste. Who can say exactly why I happen to like a particular flavor of ice cream, except from some unknown mixture of genes and experience, nature and nurture? The same has been said of one's sexual preference. Very little research has been done, no doubt because of the taboo nature of non-vanilla sex.
You may think I'm being redundant when I say that being submissive isn't sick, wrong, twisted, a perversion of nature or indicative of weakness, but it needs to be said. A lot of people know this but don't necessarily feel it; there's no point in spending the rest of your life knowing that you're submissive and hating yourself for it.
The strength and scope of these tendencies varies widely. To start with, they can be broken down into two broad categories: sexual and non-sexual. Concerning the non-sexual 'everyday' behaviors: some people like to lead, some are only happy when they serve. People at the extreme ends of this spectrum can exhibit behaviors that most people consider negative: extreme bossiness and rudeness (think overblown type A personality) or indecisiveness and an inability to stand up for oneself. Psychologists may use the terms 'submissive' and 'dominant' to describe such behaviors, and the reader should be aware that in this context they probably aren't referring to sexual D/s.
There is no clear relation between these personality traits and one's sexual tendencies - the everyday personalities of submissives span the entire length of the behavioral spectrum mentioned above.
Thus, in normal life, submissives are not necessarily pushovers - in the world of D/s, it is often heard that subs are some of the strongest people out there, i.e., strong of mind and heart. How you react to the general populace is going to be different from how you react to bedroom partners, and Dom/mes and subs are no different.
The sexual tendencies vary in strength - some people just like their partner to control a session of what would otherwise be vanilla sex or like to role-play at serious D/s; at the other end are people who are highly aroused by thoughts of being sexually dominated, and have a strong desire to experience such. The scope can range from pure D/s to including some of the more commonly associated kinks like sadomasochism (S&M) and bondage, to less-common kinks like knife play and asphyxiation.
The concept of the sub (submissive) actually being in control of an encounter (a 'scene') ranges from accepted as a matter of course to being called 'topping from the bottom,' depending on one's viewpoints and philosophies. By the nature of D/s, it is the sub's limits that are being pushed for the most part, and they are the final authority on when they've been pushed too far; any Dom/me who does not carefully mind those limits is going to have trouble keeping partners, as has been mentioned.
However, every time the sub does obey the Dom/me, they (the sub) are being controlled. Every time they let the Dom/me push their limits, they are relinquishing a bit of control. In a serious relationship, the sub is not getting every wish fulfilled at their beck and call; wheedling, whining, bargaining, or constant manipulation is generally destructive to the relationship. Being controlled is at the core of what most, if not all, subs are looking for in a D/s relationship.
The Dom/me also has needs that the sub meets - in this respect a D/s relationship is just like any other relationship, or even a close friendship. Both people have needs that the other can meet, but those needs aren't going to be exactly complimentary, so each person is (hopefully) going to do some things that they don't necessarily like to do just because they like making the other person happy.
You don't necessarily have to be a Gorean to operate without safewords, or (as I have already pointed out) to have the Dom/me truly in control. Goreans are a particular variant of the total power exchange, but not by any means the only one.
Books can and have been written on the subject, and further reading can be helpful. The web is a blessing and a curse, for while it provides a (usually) safe and (usually) private way to learn about D/s, the material that is out there tends to be produced by the people at the far ends of the spectrum. There is good material; you just have to dig for it... and for a person with no prior experience, telling the good from the bad is not easy.
Likewise for a sub with no prior experience, spotting an abusive D/s relationship can be challenging, especially if it started out healthy and has slowly declined. Power exchange provides a means of mental and emotional manipulation to the Dom that is easy to abuse. Even if a sub has consented to give up all their consent and engage in a total power exchange, they always have at least one choice left to them - they can leave the relationship. If the Dom isn't meeting their needs anymore, if they're unhappy most of the time, and no amount of polite requests have resulted in anything changing, leaving is the sub's best bet. There's no point in wasting time on a relationship that makes them feel bad and doesn't get their needs met.