How do you address a situation where you were treated unfairly at a social gathering with mutual friends?

Why do people sometimes exclude others from gatherings just because of dietary preferences, and how can this be handled more gracefully?
Do you believe that "market behavior" is driven more by cold numbers or by human fear and greed?
Is concentration, reading, and studying harder if you do not get enough cardio recess time outside?
How can giving a small tip to your gas delivery person enhance your relationship with them?