No you don't. That's the point. There are plenty of situations where a person's body, brain included, can and will be uncontrollable. It can be anxiety attacks, a chemical imbalance in the brain, arousal due to sexual stimulus, the fight or flight response, Tourette's or a bunch of other things I can't recall right now. Trigger warnings help towards managing some of these.
For instance, if someone is a Vietnam vet who suffers from severe PTSD, that person should be aware of anything that might unexpectedly trigger the disorder, like descriptions of the war in a book.