Before fetishes can be discussed though, definition is required. In common vernacular there are two ways that fetish is used, interchangeably. People use fetish when saying that a person may only gain arousal using that particular subject or object. The other usage is an object that is unrelated to sex, yet causes arousal for the owner of the fetish. Because the first is uncommon, and speaks to a deeper psychological situation, it will not be discussed.
When speaking of fetishes, it is not important to know exact fetishes, just that there are many. A fetish must be something that cannot be a normal indicator of possible sexual prowess or reproductive prowess. The average human being judges possible partners on what they deem to be indicators of this 'prowess', often secondary gender indicators (breast, hips, muscle mass and facial hair). Because we are social creatures, there is also the possibility of reproductive prowess, which shows that a partner will not only make good kids, but have the stability to raise them (money, power, motherly instinct and kindness). Considering these the benchmark which humans use for arousal benchmarks, we can determine what is and is not a fetish. Yet, when listing fetishes, some things that should be related to reproductive prowess will still be considered a fetish, because they are commonly defined as such.
It is impossible to list all fetishes, because as soon as someone attempts to, more will be hiding in the minds of those who do not wish to reveal them. Yet these deviations from the average sexual behavior will fall into groups: feature fixation, control, non-sexual fluids, appearance, food, behavior. Note that because sexual relationships are often between two or more people, all of these categories work both ways. For instance, control, which is one person having an undue amount of power over another, can be a thrill for both the one with power and the one without power. For those that found that obvious, realize that it also works the other way, with a fetish requiring no thrill on the part of one partner.
The most interesting aspect of fetishes is the level of disgust they gain from society at large. It seems that fetishes have levels of understanding, a fetish becoming more common and understood by a society will cause people to consider it not only normal, but possible to discuss (when the taboo of talking sex has already been crossed). This is a case of cognitive dissonance. Unless otherwise told, human beings consider their actions both average and acceptable. Because sex is a taboo topic, it is often only discussed when it has been breached by people who are already close. In this case, fetishes that are brought up may match, or the fetish owner may find out that their actions are not accepted by the group. Yet, as long as the topic is not brought up, and the fetish owner finds no other way to see the level of acceptance of his fetish, they will consider what they do more acceptable, even if it is still only done behind closed doors. As an example, someone who is fixated on feet (features fixation) may find it odd that someone is turned on by smoking (behavior fetish), and may find it disgusting to eat food off of someone (food fetish). Yet, the owner of each other fetish may share the same opinion of the someone fixated on feet. Each one considers themselves more acceptable, and finds no reason to accept the others. It is also possible to know of more fetishes, and to find them acceptable even if you do not have that fetish yourself.
For some the better question may be why. To some, fetishes are signs of deviancy, which of course they are. Deviance simply means difference from the cultural norms, moving away from what is seen as average. Because the average is such a small range, with most people having a feature that is indeed not average, this means that the word deviancy, which commonly has a negative connotation, deserves no such thing. When everyone is a deviant in some way, deviant is the norm. Yet this does not answer why they exist. There is no way to rationalize all fetishes, yet there are general rationalizations that can cover why some come into existence. Often this is a case of conditioning, which all humans are subject to. When we relate something to prowess, even when it is not commonly an indicator of prowess, it becomes a fetish.
Some fetishes may come into existence through taboo. In society we hide things for the sake of comfort and hygiene, and also because exposing them can cause arousal to others without trying to. So things that are kept hidden because of hygiene may become a fetish because it is so often hidden (feature fixation like feet, non-sexual fluids). It is also possible that an act may put a person into a thrill, causing a rise in the chemicals that are also used during sex (non-sexual fluids, control, behavior). Others may be a matter of ego, wanting control or wanting control of situations because you often already do, or because you usually do not (control, non-sexual fluids, behavior, food). Finally, a feature may be considered sexual simply because it was once before. Humans are creature of habit, conditioning, and vague memory, so when something was fun once we tend to believe that it will be so again (feature fixation, appearance, behavior), like having your first experience with someone who smoked all the time may lead to considering all people who smoke more attractive.
The most important aspect of fetishes is that they can cause a person to suffer due to lack of communication between peers, and leaving things unspoken. They are an important aspect of a person's identity, and when they are put down or hidden they can become a darker aspect of a person's character. On top of this, finding a long time partner, only to discover that they do not like your fetish, can be problematic for relationships in a variety of ways. This is the greatest horror of the sex taboo, and the level of moral persecution that come from talking openly about sex, arousal, and everything that comes with it. Maybe this would be a good discussion for later.