My 4yo asked why I was sad

And I didn't have a good answer. We were in the car and he just asked "Why are you sad, daddy?" and I froze. I wasn't even thinking about anything specific, just have this low-level depression that sits with me most days. How do you explain that to a 4yo? I just said I was tired but it made me realize he notices my mood more than I thought. Kinda sobering.