The most apt analogy I've ever heard for life is: riding the roller coaster. There are ups, there are downs. There are times you wish you could get off, although doing so would quite certainly be bad for your health. But remember that you always end up back at the station, breathing deep, flush with excitement, ready to ride again.
First person to reply, "Well, I don't like roller coasters" gets a bitch-slap a la ratbastid.