Should one eat fruits before or after dinner? and why?

No single answer but: Some nutritionists feel morning is the best time to eat fruit & that it is best digested on an empty stomach or with dairy, nuts & seeds or greens. These theories are based on the process of how we digest. Many say melons esp. Are best on an empty stomach. See http://www.Centerholistic.Com/index.Php/health/food-combining and see my comments. Whether its ok after a meal is controversial.

When you please: Life's complicated and difficult enough without a bunch of made-up or junk-science-based "rules". The key is getting a nutritionally adequate diet, and timing is of little importance. Most folks prefer the "treat" at the end of the meal, but it's your call; fruit is a superb between-meal snack.