We all only have so much time on this Earth. This concept doesn't tend to sink in until one reaches middle-age. I think like anything else, the less we have of something, the more we appreciate it.
A similar concept (but in reverse) is what economists call The Law of Diminishing Marginal Utility. If you love something-let's say it's cars—you'll tend to have a greater increase in happiness when you've bought your 3rd car, than when you've bought your 250th. Instead of buying more and more items with our amount of time (or life), we're constantly losing our supply of what's valuable to us. The later in life we get, the more it hurts when we've wasted what time we have left.
I've noticed something about this concept. As I've made my way through life, I see that there's a certain section of people who have reached middle-age who don't mind squandering time. It's not as precious to them as it is to me. People waiting in a doctor's office, or at an oil change station, will often sit, for an hour, and not do anything. Someone who has available time might spend hours dinking around the internet. People spend time at the beach, just sitting, doing nothing, for hours.
I can't relate to this behavior. It's torture to me. Time is far too valuable to just sit around and do nothing.
Fortunately, I love to read and write. I read when I'm waiting somewhere (thank God for the Kindle). I write when I'm home. I've recently added podcasts to my work commute. Now I can learn (mostly about writing) as I'm stuck on the freeway. It's actually made my commutes enjoyable at times and has reduced my anxiety over wasting so much of life locked in a car, sitting on a crowded freeway.
Here's my revelation: The more you love doing certain things in life (such as a craft), the more valuable time becomes to you. As you're stuck doing things that aren't what you love to do, you feel as if you're wasting incredible amounts of precious life. If you don't love to do much of anything, your value of time isn't as high as it could be.
I know when people sit around, they're often relaxing or thinking, and those are behaviors that hold value. But I don't think there's quite anything like performing an active behavior that gives you a deep sense of fulfillment.
We're all unique in many ways and love doing our own different things. If you're approaching middle-age and you don't know what your life's passion is, I'd suggest expanding your search. As life goes on, our free time diminishes, and how you spend it will in large part determine how deep of a life you'll live.