Because your system is hit with the shock of the seemingly unbearable repetitiveness involved in caring for this little creature. You feel like Sisyphus with his unending task and it's terrifying. Like being chained to a wall, or shut inside a box. You can't move freely, or do anything when you want to. Of course, the truth is we're all always trapped by the boundaries of reality -- death, the limits of time, the fact that we can't be in two places at once -- and they're all equally unbearable to face. It's just that we don't have to face them that often. We fill our lives with so many distractions that give us the illusion of immortality; the illusion that we have all the time in the world to accomplish whatever we want to. Having a baby wipes that illusion right off the face of your life, and you are left to confront the reality of the limits of space and time.