I was reading an article about aging and why that happens, and I couldn’t resist to share some paragraphs from there, I think they’re brilliant:
In their fascinating book The Quest for Immortality: Science at the Frontiers of Aging, S. Jay Olshansky and Bruce A. Carnes explain that "the price for sex is death."
While this is an extremely complicated scientific theory, here’s the condensed version: The human body is designed to live long enough to reproduce. It would take a lot of resources to keep a creature alive indefinitely. In an ever-changing environment, the variation that you get from passing along genes shuffled by sexual reproduction is a better way of keeping the important thing–the genes–alive. Moreover, any disease that appears after you’ve reproduced doesn’t matter from an evolutionary perspective. You’ve had your offspring and done your job.