The speaker's aporia manifests in the series of rhetorical questions he or she asks in the second stanza, each wondering about life's purpose. The intensity with which the speaker asks these questions shows how much he or she cared about this fallen soldier.
Immediately after a description of the sun's power to wake both "seeds" and "clay," the speaker asks "Are limbs, so dear-achieved, are sides / Full-nerved, still warm, too hard to stir?" In other words, can life really stop so suddenly after all the work that went into it? The speaker already knows the answer to this question (the answer is yes), but asks it anyway to highlight the extent of the speaker's disbelief.
The speaker's next question, "Was it for this the clay grew tall?" is a similarly rhetorical question suggesting that life's work went to waste, and that the clay "grew tall" for no apparent reason. Here, "clay" refers to mythological stories in which humans were created from lifeless lumps of clay. The speaker is wondering what the point of creating people was, if they were only going to die.
The last rhetorical question is perhaps the most powerful, for it calls into question why life would even bother existing. The speaker wonders if earth may have been better off "asleep," for it seems unnecessary and cruel that sunbeams would have "toiled" for so long just to let their creations die. Rather than providing a gentle "touch" as they did in the first stanza, the sunbeams here are "fatuous," for they only seem to lead to death. Throughout each of these questions, the reader gains a sense for how much the speaker cared for the soldier that died. If the speaker hadn't shared a strong bond with him, the speaker likely wouldn't have been as motivated to question life's purpose so intensely.