Assumptions make economic analysis easy. Economics makes several assumptions when building models. This is no different than believing that animals can talk when you read a fairy tale or mystery. You take certain things as a given.
One day in microeconomics, the professor was writing up the typical "underlying assumptions" in preparation to explain a new model. I turned to my friend and asked, "What would Economics be without assumptions?" He thought for a moment, then replied, "Accounting."
Economists take a kick in the pants from other disciplines because economists assume that everything else is held constant. Assumptions makes the analysis relevant and efficient. The following joke is attributed to Paul Samuelson whose pioneering work in economics layed the foundation for all modern day analysis.
A physicist, a chemist and an economist are stranded on an island, with nothing to eat. A can of soup washes ashore. The physicist says, "Let's smash the can open with a rock." The chemist says, "Let's build a fire and heat the can first." The economist says, "Let's assume that we have a can-opener..."