I've seen it a lot on television, in movies, and even real life: married parents that are bored with life, and have lost the lust for life that they had as a child. They take on every day responsibilities, work, raise a family, pay taxes... you know, everything a responsible adult is supposed to do. But they are empty. They secretly, though sometimes obviously, wonder "Is there nothing more [to life]?"
Some throw themselves into their passions, their vices, or their religions to find a deeper meaning of life. For some, this fills the emptiness, and they are content. Be happy for them, they are the fortunate ones. But for the rest of us, that is not enough. We yearn for deeper meaning and truths, always searching for that which we desperately need, and yet we don't have any comprehension of what we seek.
I propose this possible answer: Adventure, challenge, frustration.
Through our happier youths, we are exploring the world, discovering that which is new and different, learning that which we did not know, and overcoming challenges we rarely consider difficult later in life. We find our limitations and attempt to push through them with our newly discovered strengths. We become frustrated with those puzzles we can't solve, yet we seek the answers.
This is what most adult lives are lacking. In this modern world, the most significant adventures have already been undertaken, and are unreachable to the average man and woman. There are few unexplored places on the Earth that we can't read about or experience [in directly] through media or the internet.
The things we wish we could do are unreachable because of cost, inexperience or a lack of knowledge, greed/gluttony/hoarding of knowledge and/or materials, or other external restrictions. Or worse, our own laziness or because we can't do as we wish because we won't be home by dinner time.
As an example, I've always dreamed of flying a modern fighter jet through clouds just for the enjoyment of it. I cannot because only the military and government contractors are allowed access to those sorts of aircraft. I'm too old join the military, and I don't possess the skills that a contractor would desire, not to mention I don't have the money to acquire those skills.
Another dream of mine is to share my ideas, and my stories with others. The only means truly open to me is through the internet through journal entries such as this and posting my stories to the web. As an author, I can't get published because I can't get an agent. Why not? Is it because my work isn't good? No. It's because they don't forsee making money on an unknown author. Never mind that I might be talented, but they don't want to take the chance on me. So I'm left to my own devices, and to continue to fight the "good fight."
These are my frustrations, but they are generally beyond my control. I compensate for them in my challenges. I write computer programs to stretch my intellect as far as I can; and to learn new techniques that might get me a job. I read as many books as I can to learn new writing styles, and improve my skills. I day dream, intentionally, and design buildings and airplanes and spacecraft. I play role playing games with friends both via computer as well as face to face to seek fictional adventure and overcome my social weaknesses.
We need adventure and challenges to make life worth living. If everything were easy, then nothing would be worth the time.