I finished the audiobook of Flowers for Algernon a couple of weeks ago, and it was very well done.

As for the question, I think a lot of it has to do with connection to others. Charlie got so focused on the changes in himself as his intelligence increased that he got distracted, and lost touch with those around him. But then when he was getting ready to lose it, that refocused him on the people around him. As Khalil Gibran said in The Prophet,

Love knows not its own depth until the hour of separation.

I don't think it's a case of him necessarily realizing that his priorities were wrong before, just that they had changed as his life did. We always take things for granted to a point, and it's only when we're about to lose them that this changes. It's also a case, at least to an extent, of his being unable to do the things that had occupied him before. He can't understand scientific things he wrote, he loses his faculty for other languages. So naturally he gravitates elsewhere.

I also think there's something to this line from Dune, "Wisdom tempers love." Charlie was devoting all this time to intellectual pursuits, and it made him cynical and "worldly." In a lot of ways he lost his soul. Some of this of course was simply his functioning on another level from those around him, to the point that he was almost another species. But still, even someone as smart as he was couldn't encompass the entire world of human experience, and a more mature person would have been able to see the value in connections with others even then. Ironically, new Charlie could've learned a lot from old Charlie.