Not just the frailty of the human body, but the frailty of human morality. Who's to stop someone from going to the past to change it for their benefit? I dont think you would be able to go into the future, because it hasnt happened yet - it hasnt been set. If I make a time machine (I saw a delorean on the road the other day - kick ass!) and went to the future, I've already altered it because who's to say I'm not the ruler of the world in 4 or 5 years? By me traveling into the future, I "disappear" because I just am not around anymore to affect how things occur.