There is an unspoken code amongst friends -- one that is based on a set of shared values. Throughout the years of meeting different types of people, I've had friends come and go. The ones that are no longer in my life is usually the result of a misalignment of values.
The bottom line is this: friendship should be fun. It should be an enjoyable back and forth with neither party feeling like they have to "keep score" and both parties feeling special, fulfilled and appreciated.