Before any space lovers accuse me of solar heresy, let us consider the following scenarios as possible. Martian dust is revealed to be non-fatal towards human flesh and engineers have developed ways to keep the red particles from damaging any machines. Vast water resources are located underneath the Martian ground and science has developed a way to make the red soil fertile for plants.
NASA and other international space agencies have devised ways to reduce the amount of time to terraform a world from an extreme of 10,000 years to within a lifetime of fifty years. Last but not least, education has enlightened humanity to the need to breed amongst the stars, and has encouraged world governments to provide international space agencies with all of the funding they will ever need to accomplish their mission of resettling Mars.
But even if all of these were probable in the near future, Mars is missing a key ingredient to make it a worthy home for terrestrial life. That key ingredient missing from the red planet can be summed up in one word--resources.
Now some may consider this logic trivial or silly, but unless Mars has something to offer the solar system (or more specifically, Earth) then it will not be worth the trillions of dollars necessary to turn this barren wasteland into an Eden. Similar to the California gold rush, valuable resources on Mars would stimulate a desire to travel towards the red planet, which in turn would stimulate businesses and corporations to promote an economy upon its surface.
The moon is a perfect example of this. Until recently, there was little desire to travel back to the lunar surface, as humanity had already established its presence by landing on the white world in 1969. Despite direction from the Commander-in-Chief (i.e. Bush), NASA was still searching for a logical reason to revisit Earth's nearest neighbor.