Colonisation of space does indeed seem to be a natural consequence of our societies tendency to have a positive growth rate, i.e. for most of the world, births exceed deaths.

The planets in our the solar system are not really suitable for habitation, and would either require vast buildings, or perhaps terraforming to house a growing population. Of course our technology will need to drasticaly improve to meet the challenges that this will bring; however certain areas of our technology develop exponentially. It may well be that space based and assiociated technology could undergo the same revolution that computing has, if so within a 20 or 30 year time period, the solar system could open up to delevopment.

On a cautionary note, the very reason for colonisation, our growth rate could still be our undoing. If we don't find a way to limit our population and live within the restrictions imposed by the resources available, we will always run out of space. Even the 1-2% population growth rate of a lot of the world gives an exponential curve, eventually this would lead to a need to colonise the stars, and at an ever quickening rate. This 'wave' of expansion would eventually have to move at the speed of light to keep up; and behind the wave would be star systems, full to bursting and running out of resources....

I think we should expand, not to make our society larger, but to be more flexible and resilent; to ensure the survival of the species over geological timescales. Right now the cosmological environment we live in, could wipe out the human species. Our technology now is brittle with respect to the forces that could act upon it, such as asteroid impacts, supernova explosions, the passage of the sun though a dust cloud to name a few.
Getting into space will take a lot of thinking, staying there will really take thinking and planning.