Most people's faces are pretty neutral when they're just walking around throughout the day, so it can be really annoying when strangers see your face and tell you to smile. You're not sad – that's just your normal face! It would be a little weird to walk around with a smile plastered on, so it's likely that these strangers are just trying to talk to you.
A normal response would be to smile back or just ignore them, but perhaps the reason why you're not smiling in the first place is because you're a little self-conscious about your yellowish teeth. Not to worry because there are plenty of ways to get brighter teeth without having to shell out tons of money at the dentist or undergoing bleaching sessions. A kit like Supersmile – Professional Whitening System will give you whiter teeth in a matter of days, and you don't have to deal with messy and uncomfortable trays.
Lipstick shades can also make your teeth look brighter. A product like ModelCo – Star Smile Teeth Whitener and Hydrating Lipstick comes with a whitening product and lipstick all in one, so it's easy too apply wherever you go for a smile that you can flash to anyone – but only if you want to!