We didn't always have rainy days. A long time ago in the Land of Puget, the sun used to shine all of the time. The people became very industrious and invented all sorts of things like operating systems and strong coffee. They go so ambitious that they decided to build a really high tower, so high that it would reach the sky.
So they built it out of stone they dug out of a huge quarry, but just before it touched the top of the sky, the gods got angry, and they sent down a bolt of lightning to destroy it. And then came the rain. The gods decreed that it would rain more in the Land of Puget than anywhere else. So the remaining heap of rocks was named Mt Rainier because it was rainy-er than anywhere else. The big quarry filled up with water. When it was dry, it made a big sound when you shouted into it, so it's still called the Puget Sound today.
Then the people decided to build a huge dome to protect the people of Puget from the rain... but that's another story.