There's always a lot of serious expressions around the poker table. Sometimes I find this amusing and when someone looks at me like Vincent Vega I get the urge to smile or laugh. Is this a bad thing? What impression do you get about someones cards if they smile or laugh when they look at you? Is there a way I can use the smile to my advantage as a reverse-tell?
It depends on the smile. If it's a genuine smile I tend to think they'll be strong. With a fake smile they'll want you to think they're strong but will likely be weak. Look up Mike Caro's Book of Poker tells if you want more in depth on live tells, also look here at part 7 for smiles.
I was the same as yourself, when people would look at me I'd just instinctively smile, good or bad hand. I wouldn't place too much of an emphasis on smiling or trying not to. The vast majority of people can't read other people's body language, and even then they'll use what ever they think they see to justify what they were going to do anyway. You likely won't be playing against a body language expert.
Due to the fact the majority of people aren't body language experts, you can absolutely use it as a reverse tell. You will get people at your table who genuinely think they can read people, the majority can't. If a player at your table is saying "oh you did this, you must be strong/weak/etc", or something about a visual tell, abuse this. Try a smile, if he/she says something about you smiling you can use this against him/her. If he/she folded when you smiled, smile whenever you are bluffing.
From what I've experienced over my years dealing and playing live poker, people who place emphasis on physical tells, over fundamentals and good game theory, tend to look for a reason to justify their mistakes. Not everyone of course, sometimes you will meet people who understand body language, most of the time it can be used as a reverse tell against these players.
Smiling is a human beings way of showing they are going to move in to eat. It bares the teeth for everyone to see and telegraphs to those also eating that the person baring the teeth is about to partake. Humans are good a taking turns and will generally allow even a weaker person to share in a meal given they have telegraphed their desire to partake by baring their teeth and allowing others to see.
Dogs and animals are not this way and bare teeth to warn off not pre-warn of intent to join.
In Poker, when someone smiles, are they baring their teeth or smiling with a closed mouth? Is it their turn or yours? Are they about to be presented with an option that they may want to join in on?
Understanding the built-in smile mechanism and it's variants (show teeth versus not showing teeth for example) will help you decode what a smiling person's holding and possible intent could be.
The final thing to consider is your position in relation to the person smiling. Are you faced with a call, raise opportunity, or all-in choice to place before the other person? Are they entitled to act after your move or can you shut the round down by just calling or folding?
When someone smiles, think of the pot as a community kill. Will you let the smiling person partake?