Solomon addresses the brevity of life in these verses. While we cannot live recklessly and irresponsibly with no thought to the consequences of today's actions, we should live each day appreciating it as the gift it is and adhering to God's will for us. We should do what is right and good in the sight of the Lord not because today may be our last, but because it is what He instructs and desires us to do.
In verse 4 "But for him who is joined to all the living, there is hope, for a living dog is better than a dead lion." Here we see that the noblest, most upstanding men (and women) who die without salvation and without knowing God have no hope, but the vilest and worst of creatures have hope as long as they have life.
Eccl. 9:9 says "Live joyfully with the wife who you love all the days of your vain which He has given you under the sun, all your days of vanity; for that is your portion in life, and in the labor which you perform under the sun." Although Solomon seems to have a sarcastic attitude towards the reader here, he addresses the issue of monogamy and loving one woman for a lifetime. Proverbs 5 also addresses this in v. 15-21 instructing again that man should have one wife and not be "enraptured by an immoral woman and be embraced in the arms of a seductress." (Proverbs 5:20). We are to be faithful to our spouse throughout our lifetime.