The Preacher not only observes difficult things in life. He is not just gloomy. He also sees good things: things that can truly be enjoyed and give pleasure.
It is good to enjoy life; to eat and drink well; to enjoy the many beautiful things in life. It really is not a sin to enjoy some rest after a hard day's work. The Preacher says that it is good and fitting when you are able to enjoy your life like this.
18 Behold, what I have seen to be good and fitting is to eat and drink and find enjoyment in all the toil with which one toils under the sun the few days of his life that God has given him, for this is his lot.