Musical taste compatibility is one of those big deal things.. as in, if you hate each others music, you're less likely to get along, as it's generally very important to people. Not to say that you can't date if you have differing tastes, it's just likely for it to be a big factor.
Often the lyrics in the song, or the mood created by it, can convey the feelings of person making the mix CD to the person they'll give it to. Other times, it's just a way to show the other person that you enjoy similar things. I know I like a guy more if he can talk music with me, and trade songs that I haven't heard before and enjoy.
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"Everything counts in large amounts."