In the middle region of Vietnam, where Perry's platoon stays, for a lot of the year they're getting battered by persistent rain. Imagine the effect that would have on your mood—days and days of rain.
But rain doesn't just affect Perry's mood—it also reflects it. When something horrible happens, it often seems to be raining:
I saw the woman running across the paddy. I saw her fold backward as the automatic fire ripped her nearly apart. I saw part of her body move in one direction, and her legs in another. The woman's other child stood for a long moment, knee deep in water and mud, before it, too, was gunned down. I turned and saw Peewee walking away. The doll he made lay facedown in the endless mud. It was raining again. (17.156-159)
Gives "putting a damper on your mood" a whole new meaning. Why did Myers choose to put that detail about the weather there, after a really intense scene of a child and a woman blowing up?
Did you notice how Perry doesn't say anything about his own feelings when that happens? He just paints the picture. His comment about the rain is the closest he can come to describing his feelings. It's like the sky is crying, while Perry tries to stay numb.
Perry can't let himself get too emotional about the war. He's busy trying to survive. So the rain is sometimes a stand-in for his sadness.
And unfortunately for him, that rain just keeps coming.