Rain is basically just random noise—no pattern, no sudden sharp changes. That kind of sound (sometimes called “pink noise”) is steady and predictable, which your brain likes. It doesn’t have to stay alert for surprises.
But there’s also the context. Rain often means you’re inside, sheltered, with nowhere you’re expected to be. It creates this subtle permission to pause. The world slows down a bit, and you’re not missing anything.
So it’s not just the sound—it’s what the sound implies: you’re allowed to stay here for a while.