What makes Christopher realize he could never be an astronaut?

From the text:

And then I thought how I could never be an astronaut because being an astronaut meant being hundreds of thousands of miles away from home, and my home was in London now and that was about 100 miles away, which was more than 1,000 times nearer than my home would be if I was in space, and thinking about this made me hurt. Like when I fell over in the grass at the edge of a playground once and I cut my knee on a piece of broken bottle that someone had thrown over the wall and I sliced a flap of skin off and Mr. Davis had to clean the flesh under the flap with disinfectant to get the germs and the dirt out and it hurt so much I cried. But this hurt was inside my head. And it made me sad to think that I could never become an astronaut.