Everyone always says, "I want to have a baby." Or "I want to have a child."
But I have never heard someone says, "I want to have a teenager."
My brother and I have 8 years difference between us. I helped raise him and he's finally 18 now, managed to make it though the teens, but holy hell. No thanks. Kids are cute from infancy to around 3 years old. Then they're horrible, time consuming, annoying, expensive. By the time they're 6 years old, they've most likely said that they hate you and wish they had never been born. If not, it's coming.
Basically, you will experience worry and grief on a regular basis. Of course, you'll have the whole unconditional love thing, but don't worry, you'll have plently of disappointments on the way. Espescially as a child wants a separate identity than the narrow one you have been trying to create for them. Maybe your kid doesn't want to play sports, maybe they don't like pasta, maybe they're not as smart as you think they are.
Also, people with children tend to have nothing else to talk about but their child. When my gf and I babysit her nephew over the weekend all of a sudden the next week at work all my stories revolve around what we did with the kid. Because there isn't anything else. He's 2 and needs almost constant supervision as well as new activities due to having a short attention span.
All of that can be yours! But wait there's more! Like the average $250,000 it takes to raise a child from infancy to adulthood.
And in the end, your child may not be who you wanted them to be. In fact, they may grow up to be total douchebags who will have their pictures taken off myspace (or it's equivalent) and posted on a clothing forum as an example of what a douchebag is when someone asks.