In what way do you mean 'why do we die?'

The earth was intended to be an awesome place to live, created for us out of love by Jesus.
We weren't supposed to die, or have pain, and frankly I think it would be cool to have a pet cobra, but whatever.
Sin was introduced because we were loved enough to be given free will: ie, not be forced to do what we were told
but instead decide for ourselves. Sin was introduced, and with sin comes death.

We now die because we get old, we age, our body stops functioning correctly, we get hit by a bus like on Lost,
With technological advances, we are living longer. We now die around 80 instead of 40. Yes we live longer.

But we die because eventually our bodies wear down and stop working. Our bodies do this because sin was introduced.

It doesn't matter that you'll die. It matters where you'll go when that happens. Eternity is a long, long time.