[greens196]

Why are ethers somewhat unreactive to aqueous acid whcle epoxides are very reactive?

[Transmutation]

It is not quite accurate to say that an ether is unreactive.  It will react, in the forward and reverse directions.  Both of the reactions you describe are equilibrium processes.  However, in the case of the epoxide, a great deal of ring strain (in the neighborhood of 20-30 kcal/mol is being relieved upon opening.  The reverse process, reepoxidation, is therefore immensely energetically unfavorable, meaning that at equilibrium essentially all of the epoxide will stay opened.

Breaking the ether back into two alcohols under these conditions would be a retro alcohol dehydration.  Since heating an alcohol to 140 degrees C with aq. H2SO4 produces the ether, the ether + H2O must be lower energy than the starting materials.  Perhaps an analysis of the bond energies involved would clarify this.  Attempting to break the ether back into two alcohols is therefore energetically disfavorable, and the equilibrium will lie on the side of the ether.