Humans ask because they can. They want to know the unknown, and as we discover more, there's less unknown to us. We know what that big bright round thing in the sky is. We know what can cause people's temperatures to rise as their body gives out. We know why it gets dark in the absence of a light source. But it does make me thing a little. The only reason why we can ask ourselves questions dealing with the meaning of life is due to the fact we are capable of such questions. We're a rather insignificant chunk of the total life here on earth when it comes to pure numbers, but I'd venture to say that we're the only chunk of life that does question why. Does anybody else find that a little funny?
Now there's a few more questions: Why did the cycle start? Will it ever end? Would it matter if it ended? The bigger the picture you look at, the more you realize that it doesn't matter. But when you funnel it down to an individual level, it can mean everything. Looking at the universe it probably wouldn't matter if life ended, but to me, it would. And since it matters to me, I want to ask why it matters to me.
It's also human nature to quantify things. I'm a rather smart and clever person, but I want to know to what extent. So I make up this test that tells me I've got an IQ of 135. That puts me in the top 2% when compared to everyone else. He's got an IQ of 74, that puts him at the bottom 4%. I make 24,000 dollars a year, I'm hardly successful while that other dude who makes several million dollars a year is very successful. We then yearn to find the meaning of life so we can quantify it. Perhaps one guy decides the meaning of life is to score with as many ladies as he can. He'd then look at his count and compare it to others to see how much better he did than they. He got a whopping 34 while I only have been with three women.
There's probably many other reasons why humans ask this question, and each is as valid as the next.