C'est si simple, Raoul.

To make useful products and services to be sold to humanity for a fair price. If it's not useful it's of no value and therefore not worth the effort.

I've got a number of potential markets in mind, including a number of things that can ONLY be done in space. All involve the creation of jobs, which allows people to care for their families. All involve the creation of value, which betters the human condition. Most involve the creation of knowledge, which advances our civilization. (I've got a few desk tchotchkes in mind...)

Give me a good reason NOT to go into space. Good in the context of humanity being better off for not doing the space thing. Any argument fails in the long run from the fixed resources constraint.

I want to supply electricity from the Sun, a 4.5Bn year power supply.
I want to clean the garbage out of GEO.
I want to repair our Sun and Star watchers so that we can have a situational awareness of our galactic neighborhood.
I want to sell information on NEOs so that we can have a situational awareness of our Solar neighborhood.
I want to manufacture glass vacuum spheres.
I want to sell tugboat services for free-flyer materials science platforms.
I want to fix satellites in GEO, HEO & MEO.
I want to move cargo between the Earth and the Moon.
I want to supply pure stocks of elements to nanotechnology researchers so that they don't have to develop gobbler bugs.
I want to grow grapes in the terroir lunaire.
I want to sell information from forensic studies of GEO garbage to satellite manufacturers.
I want to construct broadcast platforms in GEO so that NEVER AGAIN must dish users be inconvenienced by rain or other inclement weather.
I want to supply in-orbit spacecraft assembly services.

Why go to space...indeed! I suggest you stop by here:

http://selenianboondocks.blogspot.com/2006/03/25-good-reasons-to-go-to-moon.html

for more good reasons.