Why do people do the opposite of what hey are told?

Answer 1

for the most part, its how the "command" is said to the person. if its in a way they dont want to hear it, then they will be more likely to do the opposite just to spite the person.

Answer 2

Why are you assuming that everyone does the opposite?

Answer 3

Rebellion. Wanting to have control over the situation. Perhaps they are angry at the person dispensing the advice....