If you use the favourites function and have accumulated a number of favourites, I am sure that, over time, you will look back and think, "Why on earth did I favourite that question?" This is obviously because there is no functionality to add a comment.
Apart from either having a piece of paper, or a textpad document that you use to cross reference the URLs/question titles with notes (tedious and liable to get lost or confused), how can I keep track of why I clicked on the favourite star?