In Flowers for Algernon, why does Winslow get mad at Charlie?

Winslow doesn't necessarily get angry with Charlie, but rather, he is frustrated by the general lack of support and participation of outsiders. He notes that plenty of people are willing to donate money.... but few are willing to donate their time.

He studied me for a moment, and then through the tiredness flashed an anger. “There are a lot of people who will give money or materials, but very few who will give time and affection. That’s what I mean.”