Ah, Kathy. I think this is an age-old question - one that we've all struggled with at some point. There are probably as many answers as there are people who have posed the question.
A few answers (some serious, some less so), in no particular order:
Because our bodies get old, get fragile, get hurt, and simply cannot sustain life any longer.
Imagine what this planet would be like if no one ever died. We'd probably be stacked up halfway to the moon!
If you're a religious person, because you're being called home to God.
None of these is a perfect answer, but as imperfect humans, it's the best we can expect.
I hope that's answered your question, at least somewhat.