Perhaps the best motivation for going to Mars is political. It is obvious that no single nation currently has either the will or the resources to do it alone, but a consortium of nations and space agencies could achieve it within 20 years.
We will that all men know we blame not all the lords, nor all those that are about the king's person, nor all gentlemen nor yeomen, nor all men of law, nor all bishops, nor all priests, but all such as may be found guilty by just and true inquiry and by the law.