Money, money, money. People-problems, personal dislikes.
Different and clouded visions regarding the company's future.
Different past experiences, leading to different priorities regarding what should be done in the here and now.
Emotional insecurity about one's future position, and by extension future social status in the company and outside of it...
Perceptions of others not pulling their weight. Goes hand in hand with lack of understanding of other people's knowledge domains, leading to wrong assessment of how hard/easy the other guy's job is.
Basically, if you want to enumerate all the causes of co-founder fights, you're not going to succeed. We're humans, sometimes some of us don't have a good reason to fight but do it anyways...
Co-founders of a business often don't know each other well before they start and when they do they often don't know each other in a stressful work environment. After you've worked together for a period of time and under much duress you find out:
It's hard to find people you jive with. I wrote to blog posts on the topic in which I advise against co-founders (despite conventional wisdom). See links below.
Typically a startup is risky, puts a financial strain on people, has them working close to each other for long hours, sometimes taking on tasks they are not good at and find frustrating. Add to that that there may be lots of expectations - reasonable and unreasonable, with hopes and dreams attached to the venture.
Aside from a marriage, I can't think of a situation that is more ripe for arguments or fights.
I split up with a co-founder years ago because I didn't like his ethics. He was double-billing. His response:
"His hour was worth at least two hours of other developers time"
This was a guy I thought was trustworthy - I had worked with him at two companies and he wrote an O'Reilly book back in the late 1990s.
We found other items we didn't see eye to eye on.
Pick your partners very carefully.
I am curious if this is a question with a purpose or the start of a blog with lots and lots of comments. You may want to put in some narrative to help with some direction.
But, there is a great deal of stress in a start-up. Some people thrive on the uncertainty, but many don't deal with it well. If you add to that the personal pressures of time away from children/girlfriend, concerns about paying your bills until money comes in, it can make for a very volatile situation.
It is important to have a way to blow off stress, and to come back to a starting point where you can think and talk reasonably, rather than having the stress being in control.
So, this volatility will lead to fights, as I am under stress, and you moved my pencil. How dare you touch MY pencil!!!!
You may think I am being unreasonable, and try to be rational, but that will just send me over the edge.
So, this is a long-winded way to say that there are many reasons to fight, as Jesper explained, but the underlying reasons may be more serious than what the fight was about.
UPDATE: Wrt how to deal with this, it really depends on what the underlying cause is. If it is a difference in vision then the solution would be different than how to spend the money you have (do we need 30" monitors or advertising).
But, as with most issues, if you work on developing active listening as well as a higher EQ (Emotional IQ), and are self-aware enough to be able to monitor what you are thinking while your anger is escalating, then you can work on fixing it.
If both of you can't do this, then learning to have a dialogue would be useful, as, in a dialogue, the idea is to safely express ideas, to defend it, to have ideas swirling around, and at the end, have a conclusion, where everyone is heard, everyone has input, and the solution will probably be a group idea, not accepting one idea or the other. This requires some experience.
If this doesn't work, then just split the work so each founder is responsible, and makes the final decision in his area, and the others just accept that, and keep your eye on moving toward the vision that started the company.
I think lack of trust and respect roots to a lot of serious-conflicts.
Most of the issues co-founders fight over, can be tamed and tackled by the other founding-team-members in a healthy way. Trust the other members, and respect them (at-least to try and understand their view-point).
I think respect is very important, you have to consider your space, other's space,... i think its a very personal thing, what is your character, true, hard-working, non-bossy, co-operative. Well again two schools of leadership, Gandhi-style or Hitler-style, both are very effective. :D Yes this question should be narrowed in its scope. Apart from quantifiable features like tech-skills, anal-skills, familiarity to some-tools, prev-exp, past-achievements, etc, ones personal level commitment is very important. Please pick a right team, it is the most important thing. I would say be careful when committing to people.
I just remember a line from "Beautiful mind", the point was...
"You gain something, when you work towards what best for the team, and what best for you".
If you are doing your best, you are happy/satisfied. If you are in a good-frame of mind you can sit and talk. A conflict might be then just a different view of the other. Sit, talk and look for a solution.
I think Jesper has mentioned some really good points and his concluding thoughts are spot on regarding sometimes not needing a reason to start a fight.
On the flip side of the coin you may hear a lot of stories about founders fighting and clashes happening along with messy disputes. I think in a lot of cases this is a healthier outcome for the startup as a whole. If there are two people or more who can not see eye to eye or have a different set of values, the sooner you break away the better it will be for the both of you.
What really kills companies is when two co-founders don't get along but just to maintain status quo do not fight either. Confrontation is messy and sometimes very complicated. This is a much more painful way of breaking apart and one which results in inflicting much greater cost on all the people involved.
In essence friction between founders is good to a certain extent. When the balance tips in either extreme that is when unpleasantness occurs and the problems begin.
Bitterness - one partner will often feel that they either do more work or aren't appreciated.
Different directions and ideas for the companies. The very differences that make founders a good team (initially) can pull them apart in the future.
Ego. Sad but true.