Your friend's who You are
Let me explain why this happens. Each individual has an image of himself or herself as a certain kind of person, with certain traits, skills, habits, possessions, relationships, and ways of behaving. As with other types of images and personality, the individual's self-image is unique, the outgrowth of that person's background and experience. Most individuals in evaluating their own images come to conclude that its not necessarily the best one around. And so they move on to conjuring ideal self images. Within the context of a social environment, these images turn into ideal social self images, ie., the way people want others to see them.
Now this is fertile ground for marketers. They can for instance, segment their markets on the basis of relevant consumer self images and then position their products or services as symbols of such self images. The consumer in all probability connects with such symbols, buys and uses such brands. This is exactly what happens when people choose friends. They choose those people as friends in whose company they reinforce their own self images. For instance, if one wants to be perceived as a 'cool customer', one tends to hang out with the airheads. As Shoba states, 'the rich hang around other rich people (or sycophants); the middle class socializes with others like them; and…you get the picture'.