You'll have to pardon Shmoop while we go take a shower. We got sweaty just watching Platoon.
When aren't the guys in Platoon sweating? In nearly every scene, somebody (usually everybody) is sweating profusely. The reasons for this are clear: the humid jungles of Vietnam, or of the Philippines (which are comparable and where the film was made) were a veritable swamp, a horrible place to be if you wanted to stay dry and cool.
The sweatiness of the soldiers gives the film a realistic feel, but also recalls the copious evacuation of that other, more important bodily fluid: blood. Sweating is the body's way of cooling itself down, but it's also a slow act of dehydration. The profuse sweating of the soldiers quietly implies that these guys are being slowly, slowly dehydrated, sucked of the life-giving "water" of their lives the deeper they go into the jungle, and into the war itself (remember during his first "hump" Taylor passes out from dehydration and heat exhaustion).