Thank... who?

Christians have said to me, "You don't have to be religious to be grateful for your life."

I've wondered about this kind of thought.

To whom or what should I be grateful for my life, if not some god?

My parents? They planned to have a baby but had no sway as to which sperm would win the sprint to the egg.
Reproduction is more of a compulsion than a choice. We are biologically programmed to reproduce and continue the species.

Should I be grateful to nature? To biology? To random chance? Mathematical probability?

It is enough for me to be happy to be alive.
Gratitude is not necessary. It is a religious foot in the door that separates the realm of reason from the realm of faith.

If you're grateful to be alive, who will you thank?