The speaker remembers a close friendship from childhood, when he and two other boys felt like more than friends, even more than brothers. Looking back, he insists on how strong that bond seemed at the time. But the last line cuts that feeling short: they were boys, and so the relationship stopped there. There is warmth in the memory, but also a quiet sadness or resignation. The dash and ellipsis in the middle suggest him pausing, trying to hold onto the idea, before admitting it could not last. The mood is wistful and a little lonely, as if adult understanding has replaced the easy, total loyalty of being young. It leaves a sense of something beautiful that belonged only to boyhood and could not be carried any further.
Boys — Summary
We were all boys, and three of us were friends; And we were more than friends, it seemed to me: -- Yes, we were more than brothers then, we three. . . . Brothers? . . . But we were boys, and there it ends.