The theme is friendship, but we might as well call it "love" in The Things They Carried, because the love shared between soldiers turns out to be the strongest form of love in the book. Girlfriends betray the soldiers, parents expect too much or too little, love of country seems a little ridiculous given the circumstances, and children just don't understand (or really try to). As precarious and screwed up as the soldiers' friendships can be sometimes, they know very well that they're the only ones who can possibly understand each other, so the bonds that they form are strong.
Friendship with other soldiers is the most important relationship in the men's lives – more important than romantic or familial ones back home. It's what keeps them going.
That soldiers form incredibly close bonds with only one person in combat conditions is incredibly problematic. If one half of the friendship is killed, the other has lost his grounding, and will never truly come back from the war.