Nicely written article, and always a good topic. But I would like to comment about what you said, that your definition of a good or a bad friend might 'sound selfish' as if that is necessarily a negative thing.
We are genetically programmed to be selfish (Egoistic Gene-Richard Dawkins and so much more bibliography). Any attempt to deny that whether it relies on pure and honest motives or evil ones is unfortunately a, quite tricky and convincing sometimes, lie. The only actual way we are able to perceive and understand the world around us is by our point of view and alone. This doesnt mean that you cannot try to understand a situation from someone else's POV (empathy etc) but it also doesnt qualify as a realistic solution. And simply because no matter how advanced or experienced your mind can be (rational knowledge) you cannot compete with the experiential understanding of the other person. Each and every one of us sees the world in a specific way every second it passes based on a series of different factors. So you can never fully perceive the entire emotional, cognitive and biological spectrum of another person's actions.
The reason I said that is because its where my definition of friendship comes from. For me, a good friend is a person you share common beliefs, interests and experiences with but when the time comes to argue/fight/disagree over something he consciously chooses you (the friendship) over his ego because he simply understands that you can both be different and yet somehow connected.
Sorry if It was too long haha, good article anyway :)
RE: You might not be such a good friend as you think you are - A simple philosophy of friendship