Ben Horowitz writes:
Most business relationships either become too tense to tolerate or not tense enough to be productive after a while. Either people challenge each other to the point where they don’t like each other or they become complacent about each other’s feedback and no longer benefit from the relationship. With Marc and me, even after 18 years, he upsets me almost every day by finding something wrong in my thinking, and I do the same for him. It works.
This has worked for me in my personal relationship over last xx years, and is one of the few things I seek in any good business relationship as well.