Why did Robinson Crusoe name a desert island?

It's called "Island of Despair", starting from the second paragraph of chapter 5, "Keep a diary"

My diary starts from that terrible day on September 30th, 1659.

September 30(th)

I'm Robinson, and our ship was caught in a terrible storm at sea. ..........

I don't know the name of this island. I call it the island of despair.