Startups often struggle because of how they are set up. Too often founders will treat a new company like a budding relationship: you get all excited about your idea, you seem to agree on everything, and then you move in together. Relationships built on that kind of whirlwind courtship often end up in failure, and startups are no different. It is critically important for co-founders to have the detailed conversations about their business, their working relationship, and many other topics that they will have to deal with as the company grows. In fact, we have discussed this here on a few occasions (see Founders Series Parts I, II, and III.
One of the people who knows this best is Dharmesh Shah, successful entrepreneur and c0-founder of HubSpot. He recently published a list of the most important questions that co-founders need to resolve as soon as possible when starting a new business. If you are in the process of starting a business, stop what you are doing and read it.