For the short term:

Bragging rights.
Improvements to quality of live with space technology.
Military strength with space technology.
Research that can only be carried out in space.

For the long term:

Resources, Earth will eventually run out raw materials.
Overpopulation, impossible to fit everyone into this planet anymore.
Survival of our species. We're all the eggs in one basket, if anything happens to our precious earth it'll be the end of humanity. (meteor, wandering black hole, sun going supernova, etc)

It's hard coded in our DNA, no matter how hard we bomb the shit outta each other, deep down inside humans have this WILL to see the continuation of our species. To see this through, space exploration is the natural option to take, in fact it is the only option.