Is there a scent that makes you laugh or smile? 

How about the smell of cotton candy at a Theme Park? If you remember good times, laughter and being care free; that aroma may trigger some laughter and feeling of joy.

Your response to a scent can be linked to memories or in some cases situational. What if there was behind closed door marketing meeting with the CEO of a snack food company...?

You  nonchalantly walk by and can smell the odor of marijuana wafting from under the door. That would indeed elicit a laugh because of comical visualization executives using grass smell to promote snack food.

How about the smell of bacon frying in a vegetarian restaurant?  That could be funny, or disgusting...the choice is yours.

Ever heard the expression..."that really smells funny?" That may also mean not so good!

A persons reaction to scent is very, very personal; based upon experience, preference, the situation, place etc. For example:

You may love the aroma of a Thanksgiving turkey roasting in the oven, but if you ate too much and feel sick,that aroma could be a huge  turn off...at least for a period of time.

I remember as a kid when my friend held up a sock, smelled it and then wrinkled his  nose in disgust while saying ugh! 

We would laugh and try to run away from it. There was a lot of laughter and horseplay over a smelly dirty sock:)

 Definitely not something that you would want to hang on the mirror of your car. Even though thinking about that makes me laugh. Thinking about that sock or smelling it are two different forms of humor... I prefer thinking.