What does it mean if your talking to a guy and he just smiles at you?

When a person smiles at you, it really depends on the situation to determine the true reason behind the smile. If it's a person you have never met before, their smile could be caused by the way you are dressed, the way you walk, something that you're doing, etc. You might make them recall a fond memory, or you might be someone they've never seen before and they like what they see. They could also find you quite attractive. If someone you know by face but are not close with, he/she could simply be acknowledging your presence and saying a Wordless hello to you. A smile can often tell more to the receiver than Words, or it can completely confuse the receiver and cause him/her to ask this philosophical question, just like you did. The circumstance, the time of day, the situation at hand, the quality of the smile, the type of person who gives you the smile (usually judged or misjudged by physical appearance and manner if you don't know that person), the place it occurred, all need to be taken into account before trying to determine why that person smiled at you. There's no simple answer to the mystery of a smile and oftentimes, your best judgment can determine why you were smiled at. If you still can't understand why in the world a stranger smiled at you, if I were you, I'd accept it as a gift and give one back in return. Then, I'd go on with my day with a smile.