I am not going to try and surmise Solomon's reasons for doing them. Instead I am going to apply this to my own life with that same retrospect. I can ask myself the same questions. What have I done in my own personal life for the sake of bringing myself pleasure and how long might it have lasted? Perhaps you need to evaluate your life in the same way. When we take the time to really sit down and evaluate the reasons behind the things we do in this life, I'm afraid we will find that they are often for our own selfish pleasures. We'll perhaps realize how often that God has been totally left out of the picture. For Solomon, it's as if the light bulb has finally turned on and he is now seeing things from God's perspective.
I believe it is a healthy thing to look back over our life and our accomplishments and understand the motivation behind them. Did we do things for our own benefit or did we include God in every decision we made. If you are like me, I think you'll find much of what you've done has also been for the wrong motive. Perhaps if we take more time to stop and evaluate our life along the way, we will realize how selfish we can be. We often forget to bring God into the picture and our decision making process. I think it is very healthy to do this on a regular basis and then correct the things we see as we move forward in life with God's help. I want to be able to say that I have lived my life to its fullest for the cause of Christ and with no regrets. The only true way to do this is to have regular times of introspection and evaluation.