It would seem that at the moment the only significant pull-factor for human space travel is scientific curiosity. It can for instance be argued that there is no economic incentive for humans to go to Mars and that there probably won't be for a very long time. The same and more applies to inter-stellar travel, where besides the formidable technical challenges involved, there would seem to be even less of an economic or other incentive besides scientific exploration.
Now, considering all the economic and technical challenges, what would constitute significant pull-factors for human space travel such that progress would be accelerated. For instance, what must be discovered on Mars (like certain minerals for example) that would get humans there in a much shorter period of time than would have been the case otherwise. More hypothetically, if Mars was a more habitable planet (like a second Earth) would humans be there already?
What about inter-stellar travel? Say hypothetically speaking, that an Earthlike paradise is discovered in the Alpha Centauri system, how big of a pull factor would that be for investing in inter-stellar propulsion research. Or say, two or three Earths are discovered how would that accelerate the pace of developments. What would it take, in terms of pull factor, to make people go from saying "It can't be done" to "How can it be done"?