(This was originally posted at my personal blog, Fraggmented, but I don't think it quite found its audience there. I repost it here unedited.)
Day One: You settle into your apartment, and you each agree on an equitable share of rent and utilities. The two of you sign the lease, taking an equal share of responsibility for making the apartment work.
End of Month One: Your roommate explains that he's starting his own business, which should bring even more capital into the system. He explains that for a little while, he needs to reduce his contributions to rent, but that this will pay for itself in the long run. When asked about the short-term, he suggests that the household just "run a deficit" for a little while. He shows you his business plan, and you agree.
End of Month Two: You find out that the business plan your roommate had was just a bunch of random guesses, and he's not spending any of the money he withheld on his supposed new business. When you confront him on this, he gets very upset that you're calling him a liar, and insists that you're just greedy and want his money. He tells you that if you want to balance the household budget, you could maybe cut the food budget (since he's eating out all the time anyway) and cut back on your heart medication and maybe turn out the lights when you leave the room. After a five-hour argument, you agree to turn out the lights when you leave the room. He does not concede anything.
End of Month Three: You're now having to kick in extra money to make ends meet on the household budget. Your roommate responds to this by suggesting that since you're clearly "hoarding" money, he should respond by reducing his contributions even further to get his business going and make the household function again. You point out that he hasn't spent any of the money he already has on his business. He accuses you of hating babies and caring more about homosexuals than you do about him. You insist that he start contributing his fair share again. He suggests that you look at the bills again next month to see if things change. When you refuse, he threatens to stop giving you any money at all and to let the landlord evict the pair of you.
End of Month Four: Your income is now stretched pretty much to its limit, as your roommate isn't even giving you all the money he promised to give you. When you point this out, your roommate suggests that you could probably make up the shortfall by selling your car, as he knows a friend who's in the market for one. He admits that his friend is aware of your financial troubles and won't offer too much, but some money is better than none, right? You point out that this is a short-sighted plan that will wind up costing you more in bus fare than it saves in gas, and he tells you that beggars can't be choosers. Reluctantly, you agree to sell your car.
End of Month Five: Your roommate is now driving your old car. He doesn't see why you're upset about the agreement he had with his friend to share it if you wound up selling it.
End of Month Six: Your roommate refuses to pay the phone bill, because he no longer uses the landline. You point to the agreement you signed with both your names on it. He shrugs. You pay the phone bill.
End of Month Seven: Your roommate finally agrees to pay the phone bill. A few hours later, you hear him in his room violently arguing with himself. A few hours after that, he comes out of his room and explains that he can't pay the phone bill after all. He appears to have hit himself a few times.
End of Month Eight: After another five-hour long argument, your roommate finally admits that his business plan has a few problems. He agrees to create a new business plan. The next day, he shows it to you. It's the same business plan he showed you seven months ago, only with pie charts and a new name.
End of Month Nine: Your roommate mentions that he got another angry call from the landlord yesterday about the rent, and he's not happy about having to deal with it. He suggests that you maybe stop spending all your money on drugs and prostitutes and work on making the apartment a better place again.
End of Month Ten: You find out your roommate has a $1000-per-week drug habit and has been bringing hookers into the apartment when you're at work.
End of Month Eleven: Your roommate starts pointedly reminding you that the lease is up soon, and that he's not sure he can support a deadbeat like you anymore. When you point out that he hasn't actually paid you his share of this month's rent yet, he calls you greedy. You find him in your room a few hours later with a tape measure.
End of Year: You start looking for a different roommate. Your roommate insists on being present every time you talk to someone, and calls them all communists.