Who seems to be missing in this book?
I'm sure this answer is pretty much wide open to personal interpretation - but I think when I read this book, the one thing that rings the most loudly and truly is Morrie's call to positivity and love - which he shows by surrounding himself with people. His authentic relationships (with family, friends, even his careworkers) are the most affirmative action that Morrie does to show he practices what he preaches.
Therefore, I always thought it painted a pretty clear void in Mitch's life - that he is so isolated, even to the point that he doesn't recognize loneliness. In fact, Morrie is the first person in quite some time to whom Mitch makes a commitment - and actually holds up his side of it - by coming religiously to visit. Everyone else is put on his personal agenda as "to-do" points - and his wife falls off almost completely.
It is likely that this is the exact reason Morrie has such a profound impact on Mitch. Morrie's advice speaks to exactly what Mitch needs more of in his life - cultivated, personal, loving relationships. So in answer to your question - "Who" I would say the answer, for Mitch, is anyone.