Everybody here has been hypnotized. You know when you get too caught up in a video game or a TV show and you lose track of time? That’s hypnosis. Hypnosis is neither a state of relaxation nor some-weird-mind-control-thing, it is a state of extreme concentration to the point that the person stops paying attention to anything else.
The most widely known form of hypnosis is the kind that is intentional, when there is a “hypnotist”. In this case, in order for the subject to be hypnotized, he must both be willing to be hypnotized and unafraid of hypnosis. People who do not believe in hypnosis are also not able to be hypnotized.
To answer the question, the reason you are not able to be hypnotized may be simply because you are thinking too much about how you are not able to be hypnotized. Another reason is if you are thinking about “what if he makes be do [insert embarrassing stunt]” and are afraid of being hypnotized. The whole way hypnosis works is by having all of your concentration focused on the hypnotist, if you are paying attention to other stuff such as fear, then hypnosis is impossible.
Some other things:
A large percentage of stage hypnosis is just that: staged. Online hypnosis videos are even more likely to be fake. Anything that involves the stereotypical swinging watch is fake, as is anything about instant inductions (real hypnotic inductions take a lot of time).