That's putting it very simplistic by someone who obviously does not understand.

Here's a basic look at how it works

Let's pretend you get to work late because of construction on the road and your boss jumps you about it and threatens to fire you if it happens again. You try to explain, but he ignores you and walks off.
You can:
a) get mad and quit
b) get mad, but keep your mouth shut but don't really pay much attention to what you are doing and possibly make a lot of errors
c) wait for him to calm down and try explaining again
d) start looking for a new job and quit asap

Now - depending on what would make you happier (to keep the job, or find a new one) eitger c or d is best option but a or b are the gut reactions ...

You cannot control the road construction or traffic
You cannot control your boss reaction to you being late nor his action of beligerance
You CAN control your reaction to your boss

By controlling your reaction - you will control your overall mood and behavior, thoughts and feelings

Our decisions have a lot to do with our moods even though it's not an obvious coorelation