Apple - Why does iTunes continually ask me to authorize my computer?

In the end, deleting all the offending track and re-downloading it was the only thing that worked. Why I didn't think about this at first, I don't know, but it worked!

JUST found the solution to my nearly year long battle with iTunes and the: You are not authorized. You're already authorized issue. Just adding this to spread the word that Ctrl +b -> Edit -> Preferences -> Advanced -> Reset Cache worked for me.

Every time iTunes started it thought it was freshly installed and prompted me to go through the tour, set the default music player, and authorize (each time saying it was already authorized). It wasn't just one song, it was about 50% of my music and 100% of my movies. I had already tried- deauthorizing then authorizing, uninstalling, reinstalling, deleting that SC- folder and restarting -- nothing worked until I cleared the cache.