Explain the principle behind chemically reactive and inert molecules

Electronegativity.
Molecules with atoms which possess an almost full outer shell will always be seeking to draw in the extra electron(s) to fill up their outer shells.Likewise molecules with atoms which have an empty outer shell will be seeking to lose their electrons to obtain a full outermost shell.
Many reactions in the world, both naturally occurring and synthetically occurring exploit this property of certain molecules, atoms or ions. An electronegative molecule will be drawn to a region of high electron density, and an electropositive molecule will be just as willing to give up its electron share. Drugs, synthetic molecules and biological molecules are designed in such clever ways that they will react in a certain course if bought into contact with other reactive molecules.Atoms which have a full outer shell of electrons are stable. They won't be attracted to either electron dense or deficient regions: hence they are inert.
When studying Chemistry, it is crucial to understand which molecules are electron dense or deficient. It will be these molecules or atoms which will be the most reactive, and we can therefore predict how certain moelcules will react with others on this basis.