Why does Holden say that he misses people at the end?

At the end of the story, Holden has undergone psychoanalysis.  He tells us in Chapter 26, that his:

"Psychoanalyst guy they have here, keeps asking me if I'm going to apply myself when I go back to school next September." (Salinger)

Holden's admission of missing people is a sign that he is getting better.  He has been in a mental institution and now, he is anticipating the future, the next school he will attend, how he will do once he gets there.

And, now that he has undergone treatment for his depression and can reflect with accuracy on his experience at Pencey Prep, he realizes that he actually liked Stradlater and Ackley.