Why do we eat startersbefore the main course and dessert after the main course?

Long standing social convention. Basically, the starter is a taster, a light thing to open up the meal, get your stomach ready, really, usually small and/or vegetarian, the main is the main point to the meal, and dessert is last because having something sweet denotes to your stomach that you are done eating. It has to do with how we like to eat, both mentally and physically. Apparently in france they have meat first, then salad, then dessert. It depends where you are, really, but in north america it's starter, main, dessert, and that's what most restaurants and caterers follow.