Why doing a Ph.D. is like bungee-cord jumping
Like bungee-cord jumping, doing a Ph.D. only appeals to a few people: for the rest of humanity, it is a skill not particularly worth having. You start the Ph.D. at the top, jumping off into the unknown. Until you hit the bottom, you do not quite know if you are actually ever going to return to terra firma (and for a few unfortunate individuals, the trip is one way). People walking past look at you descending fast, wondering why on Earth you are doing such a stupid thing. At the bottom of your descent, your self-esteem is at rock bottom, your fear level is at its peak, and you wonder why you took the leap in the first place. Finally, you find a topic, and slowly you return, against gravity, to the top. The bungee cord is your advisor, who will let you fall, but (hopefully) has a firm grip, and when you reach bottom, jerks you back up again. When you finish, you have learnt something about yourself, and you emerge stronger for it.
...your head knows that to jump off is madness, your heart tells you to go for it.... (quotation from JoJaffa's bungee jumping page)
Incidentally, I have never been bungee-jumping, and I don't intend to, ever.