When deciding whether to have a personal relationship with someone, you probably (implicit or explicitly) weigh the benefits and costs of such an investment of your energy. For people you have strong relationships with, there are probably loads of benefits you reap from that investment which outweigh the costs (even our closest friends have one or two annoying habits!).
Here’s when the cost-benefit pie breaks down for me: deal breakers. Deal breakers are a handful of things that automatically overrule the cost benefit equation. For me, one deal breaker is cocaine. No, I don’t spend a lot of time around coke addicts, but having spent four years in high school, drugs like cocaine pop up. People make choices. If someone wants to do coke from time-to-time, it doesn’t matter if that’s the only "negative" — I don’t want to have a relationship with that person.
It struck me that people who have deal breakers are probably less loyal friends. If you had to make a list of the most important characteristics in a friendship, where would loyalty fall? For me, it’d be on the list, but not at the top. I think you should stand by someone in thick or thin, except when the thin can drag you down too. (In the case of a drug addiction, I would seek professional help for my friend, and then get out of the way.)
Do you have deal breakers?