There are some questions that are sensible to ask, such as: 'What time is it?' or 'What does this machine do?' or 'Who killed that sheikh?' or 'Is there life on Mars?'. There are others that make no sense, such as 'What colour is Tuesday?' or 'How many apples make a banana?'. 'Why should we die?' falls into the second category, I fear. See Richard Dawkins's River Out of Eden in which he discusses this sort of question in relation to human existence.

Posing the question presupposes that there is a purpose to the existence of any individual over and above the propagation of his/her genes - which there ain't. As mentioned above, humans (and even their eggs and sperms) are just their genes' way of making more copies of themselves. See Dawkins's The Selfish Gene, widely accepted as the basis of modern genetic and evolutionary thinking. (As you can tell, I'm something of a fan!)

So that's the philosophical answer (IMHO!)

On a pragmatic level, I agree with the ecological and evolutionary answers given above. If we all stayed around after we'd done our bit for our genes there'd be less to go round for our offspring and so they'd be less likely to survive, once they'd become independent.   Furthermore, without death, natural selection doesn't work and we wouldn't have evolved in the first place.

Of course, if you believe that we were all specially created and there is a purpose to our existence beyond making more genes, you won't accept any of this. It must take a massively strong faith to believe all that stuff - but then I suppose faith is what religion is all about!

Steve