Some of these ideas are stated so often in books about sex that I sometimes start to think that they must be true.
There is a relentless human urge to explain things, and if we don't know the answer, we can always make something up.
Many attempts to "explain" sadomasochism and fetishism suffer from one major problem: an author will give one plausible explanation for leather/rubber fetishism, another plausible explanation for sadomasochism (or even one plausible explanation for sadism and another plausible explanation for masochism), and completely ignore the fact that leather/rubber fetishists and sadomasochists are to a large extent the same group of people. In which case there should only need to be one explanation for all these phenomena.
This is sort of the dictionary definition, so in a sense it has to be true. Supposedly a leather fetishist is attracted to leather instead of people. But as far as I can tell, leather fetishists are attracted to attractive members of the opposite sex (or the same sex, if they are homosexual) wearing leather. And they are quite happy to perform normal sexual intercourse on someone dressed this way (or indeed on someone not dressed up).
I don't think you have to be a fetishist to be scared of the opposite sex, especially if you are attracted to someone and somewhat uncertain as to what they think of you.
Books that push this line sometimes give examples involving very unusual sexual fetishes, like the man who has to see women sneezing. But in the real world, certain fetishes are much more common than others. Sometimes a person can trace their sexual preference to some early incident in their life. But this fails to explain the frequent occurrence of certain fetishes overall. There must be some predisposition towards these fetishes, and this predisposition still needs explaining.
One can get into endless arguments as to what is a mental illness, or even if there is such a thing. Certainly there exist many sadomasochists and fetishists who are obviously sane individuals.
If someone turns up at a psychiatrist's office complaining about their sexual preference, does that mean that that sexual preference is a mental illness?
It could be that sadomasochism is just a normal variation in sexual preference, and is no more a sign of illness than many other variations in human personality.
Well actually, pain and pleasure are opposites. Also, there are many forms of masochism that don't involve any actual physical pain, so any explanation that depends upon a relationship between pleasure and pain fails to explain the whole phenomenon.
This explanation is a consequence of the Freudian view that mental problems in general are caused by subconscious thoughst and suppressed sexual desires. There is no real evidence of the validity of Freud's view, and it is not very testable (until such time as the "subconscious" can be observed directly in some way).
The suppressed response theory suggests that this explanation is quite wrong, because sadomasochism and all related forms of sexual arousal depend on a temporary suppression of normal instincts, desires and preferences. That is, masochism is exciting because normally we don't like being hurt, and being dominated is exciting because normally we don't want to be bossed around, and sadism is exciting because normally we don't want to hurt others, and so on.