Happy or Right which is it??

I use to work for a dentist who was a total idiot. He said stupid stuff all the time and he told lies ALL the time. 

Like the time he tried to convince me that of all the dentist in our area he had been chosen to do a root canal on a tiger. When I asked to see pictures of proof he could not find them. 

Hmmm...guess I just thought you'd frame proof of that and hang it next to the 2 pound trout that is the office centerpiece.???

But he did say 

You can be right OR you can be happy but you can't be both. 

Is this so?   I think it may ha e some truth to it.