It is a mystery, like lots of other phenomena that science fails to explain. Personally I think all those creatures are much more intelligent and aware than we give them credit for. Viewing these creatures as simple automatons is as silly as viewing humans as such.
Wonderful! Watching one's closest people age is so cool! I love looking at my wife (we've been married for 20 years) and seeing how she's changing through the years, her eyes, her skin, her figure, it's fascinating. Same for the wrinkles on my mother's hands, or even my own.
For me, there's nothing scary or sad about growing old and then dying. It's natural, it's beautiful, it's just great the way it is.
That's one way of looking at it. Parenting had changed me profoundly for the better, and even though I made horrible mistakes as a parent (as most people do I guess) I've also managed to significantly improve many shitty aspects of my personality.
I think in parenting most of all you learn to give, without hesitation, without thinking too much, you just give to you kids whatever they demand, out of your unconditional love for them.
Besides, if you wish to become a happier person, kids are such an amazing source of joy and hope! Oftentimes kids can see things much more clearly than adults, they have their own kind of intelligence and reasoning and we adults have much to learn from them.
There are many more "selfish" reasons to have kids. If you do it right and develop affective relationships with them as they grow up, they'll take good care of you when you grow old and frail, and your grand kids will bring you even more joy and entertainment! I don't think it's much fun to be old and alone...