There are only two reasons I’ll reread something. Either to see how I’ve changed since the first time, or because I feel I missed something. Age has a lot to do with the former, and insight with the latter. Both may be closely related, and accompanied by a sense of nostalgia. Fond memories alone aren’t my reason to read.
For example, I first read the Dune series (only through Chapterhouse) when I was fifteen years old. I thought I might have a different view in my late twenties, I was right. The backstory felt more real, my connection to Paul was more distant, but the urgency for his quest to succeed was stronger. Time changes us all.
By comparison, I reread “Time Enough for Love” by Robert Heinlein at least three times. It isn’t to see how I’ve changed, but to find other meanings in the book I hadn’t seen before. The characters, settings, and events have more meaning each time. Maybe lots of books are like this, but I reread so few I don’t’ have enough frame of reference.
Both reasons for me to reread are connected. I could even argue they are different facets of the same idea. Age and insight do go hand in hand. For me the difference is inward versus outward. Can I see more of a change in myself, or recognize more depth in the material? Both answers should be yes, but the devil is in the details.