Why are some so sensitive to caffeine?

Question

What reaction is going on in the body that causes this?

Answer 1

Biochemical Individuality.  Diet, genetics, other chemical exposures, etc. create a large variation in detoxification activities in humans.  Those who are slow to detoxify caffeine may experience much stronger effects from it.

Answer 2

Caffeine is a stimulant. It is added to certain beverages for that fact and is a natural part of coffee and tea. the old advertising slang of tea revives you is perfectly right. It does because of its caffeine.

Some people are just not tolerant of it. They are, simply put, allergic to it. That's all.