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?
From Favorites improvements - search, categorize, personal tags, add note, favorite answer, you might find that this answer could help you tremendously.
ᔕᖺᘎᕊ has written a very useful stack app. I have been using it for over a month now, and have found it invaluable, and tbh, I don't know how I functioned before installing it.
The stack app is available here: Favourites Enhancer: Tag favourites and add personal notes
You will need to install Tampermonkey (or Greasemonkey), and I've tested it on Opera and Firefox, and it works fine on both.