Yeah, four days of people who share a hobby and a passion, having fun, throwing around terms like "spell DC", "saving throw", "success with a raise", etc. without confusing others.... then getting back to your normal job, your bills, your house cleaning...
It's not sadness, it's certainly not legit depression (I've been there. Big difference), it's just a bit of a let down coming home. There's just not the same vibrancy to walking out the door when you leave the convention.
I know that'll change soon. It just takes a bit to re-level after a vacation. But yeah... it feels just a bit down, just a little mundane. Normal life is not as fun.
Here's to looking forward to next year already. It'll slowly simmer from January on, then ramp up in July. That'll be fun. Until then... normal life.