There’s no one specific cause that stands out as common. All happy families are alike; each unhappy family is unhappy in its own way.
What IS common amongst terminal cofounder conflict is a series of ruptures in the relationship without any corresponding moments of repair. Over time the ruptures build up into complete relationship breakdown over an issue that seems trivial. People gradually work themselves apart and can’t come back. Some of my biggest failures as an investor have come from not understanding this dynamic.
What I’ve observed is that great cofounder relationships display a pattern where conflict is followed by a moment of restoration so that a strong positive relationship builds over time. The biological analogy of this is muscle hypertrophy. You repeatedly lift something heavy and your bicep muscle fibers are damaged. Under the right conditions of rest your body repairs the damaged fibers by fusing them, which increases muscle mass and so you get stronger over time. Or you don’t have a repair cycle and instead keep lifting heavy things without rest until you eventually get a traumatic failure and can’t lift anything for a long time (if ever).
In the stress of a startup conflict is inevitable, and to a certain extent at times it’s also required for progress. So the insight here is not “great cofounders have zero conflict”, but rather “great cofounders follow conflict with moments of repair”. If you consider each conflict a small tear in the relationship with your cofounder then for every tear their needs to also be a compensating motion of repair.
What I’ve learned to do now as the investor in the loop is to help cofounders notice early when ruptures are occurring without repair. Most of the time merely drawing attention to this dynamic is enough for them to course correct.