I learned this lesson at my expense.
I was enjoying a MURALES beer "Liberos Animos Sardos"....
...with my best friend, laughing at our funny conversation. The beer was taking effect and things seemed perfect.
"I have a business idea, why don't we become partners?"
And that was exactly how our friendship continued on a parallel path, that of business.
We were excited, we trusted each other and we had a very clear dream of where we wanted to go, so we started working on our project.
Very soon little misunderstandings began to appear, yet we ignored them because we were "best friends".
And so it was that we took a dead end in our business while investing more and more time and energy in the project.
At some point, difficult conversations had to be done but we never did. We had no formal agreement except to be best friends: it was too late to even talk about it ...
Problems and misunderstandings began to develop rapidly and eventually the beers ended. Then the fun is over. This negative thing between us entered the business which led us to failure.
Friendship is over.
To answer your question: what should an entrepreneur never do?
Do not embark on entrepreneurship just for friendship! It is incredibly easy to get involved for the wrong reasons.
John D. Rockfeller has a nice quote on this.
"A friendship built on business is better than a business built on friendship"
Running a company and a friendship is a very complex thing: it requires very mature people for it to work.