Venture capitalist Ben Horowitz writes in The Hard Thing About Hard Things how he and fellow venture capitalist Marc Andreeseen have managed to work effectively together across three companies over eighteen years:

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 eighteen years, he upsets me almost every day by finding something wrong in my thinking, and I do the same for him. It works.