If you think you're having a bad day, then you've obviously failed to ask literally anyone around you how their day is going. And if you did take the time to ask a stranger what's up, you just knew the response wouldn't be jolly.
I'm not a mind reader or anything — I just know that humans tend to focus on the negative. Unless something truly and hugely amazing happened to you that day, you'll always focus on the reasons why you shouldn't smile, because lord knows there's a lot of 'em.