In the second stanza, the speaker first tells us that it is important "never to forget / The essential delight of the blood drawn from ageless springs / Breaking through rocks in worlds before our earth" (lines 9-11). To me, it sounds as though the speaker is celebrating the miracle of our continued life: each of us has blood drawn from ageless springs; it is difficult to fathom humanity's beginning or end.
Then, he cautions us "Never to deny [by forgetting] its pleasure in the morning simple light / Nor its grave evening demand for love" (12-13). The word "its" seems to refer back to the "delight of the blood" from the sentence before, so this seems to suggest that we should also not forget the pleasure of the blood in the simple morning light. This makes me think of the peaceful feeling one might get in the morning while watching the shafts of golden sunlight enter through one's window. Further, just as we should not forget the pleasure we feel in the morning light, we should also remember how seriously at night we long for love. There is a certain necessity to each of these feelings.
Finally, forgetting these feelings would "allow gradually the traffic to smother / With noise and fog, the flowering of the spirit" (14-15). If we forget the beauty of being alive, our spirit will not be able to flower. It will have been smothered by all the "noise and fog" that allow each of us to forget.