I don't think we choose our friends. They are people who cross our path in life whose company we like. Often there are common interests, but they don't always perfectly align, but they are our friends because they possess some intangible quality that we find appealing. And, like anyone in life, if they treat us badly, even after years of friendship, we let them go. So friendship isn't necessarily a life-long thing. We all change, and the person we thought was so fun at 18 can seem not so much at 30 when we have gone through adult experiences that shape us.