... not exactly what you think it might be. It isn't simply a selection of music you make for - or receive from - a friend. A collection of music (specifically on cassette and created by dubbing music with one's fingers on the record + play buttons) is more a mix of time and everything the individual attaches to it.
Music is an aural text of our lives. Why do you think some songs - regardless of melody, rhythm and dynamics - make us cry, laugh, scream, or move us to dance? Sure, hearing a song for the first time can make us do all of those things, but that is because it has become the present text onto which we sill subsequently connect our experience. Familiar pieces of music inevitably evoke the environment in which they were first heard, or the general era of our lives in which they became part of our sonic memory: how old we were; if we were with friends or alone; if we were happy or sad; what we were going through at the time; even what the weather was like. We can remember tastes and smells from music. We can remember imagery. And the more we listen to a piece of music, the more diversified and rich the associations become.
A mixtape - not manufactured, but hand-made - is thus a visceral record. Although an artist will release a recording relating his/her experience through song, the mixtape is the listener's record, adding a personal element which strengthens the magnetic tape itself. Like a handwritten correspondence, the music exists between loops, waves, and angles of the letters. If you look hard enough, you can strip away the alphabet and see thought; if you listen hard enough, the sounds aren't solely musical: they are emotion. They are the steps of our progression forwards, and the balance we need to make them.