At first, the poem reads like a simple snapshot of a hot summer day. The sun is out, the bugs are loud, and the landscape is full of yellowish greens. People crowd the shore, the sea breeze blows, and children play with so much energy that the heat hardly bothers them. The speaker sounds cheerful and a little nostalgic, noting that the child from spring has grown older and now enjoys summer "with absolute vigor." Summer here seems to stand for a lively stage of life.
The last line changes the tone. After calling summer lovely "in life," the speaker adds that it is strange "to not be alive." That quiet remark hints at death, and it makes the earlier scenes feel more precious. The mood begins warm, noisy, and carefree, then ends on a thoughtful, slightly wistful note. The poem seems to say that part of what makes summer, and life, so bright is that it doesn't last forever.