1) Because it causes most of us to lift our eyes up from the ground and look outwards, something we humans have done for a long time.
2) If the first exodus from Africa had never happened we would never have been so successful a species as we are now. We also would probably not be here now.
3) Because it is the highest cause to which we aspire.
4) Because it is exciting for the human spirit.
5) Because it challenges us to push the envelope. If you don't push the envelope you stagnate and eventually retreat to insignificance.
6) Because of what happened to nations in the past who turned their backs on exploration, two names Portugal and China show how a nation can go from preeminence to almost irrelevance in a short period of time.
7) Because it is the greatest experiment that requires all nations to participate together. What better way to come together?
8) Because we will eventually, one way or another, need to defend ourselves against some solar system detritus. Only a strong presence in space can do that.
9) Because robots cannot be inspired.
10) Because sending my 35mm Instamatic to the Grand Canyon does not a vacation or expedition make.
11) Because I do not wish to spend any more of my life in virtual reality. I want real reality.
12) Because sending robots will produce new technologies in electronics, materials and a few other disciplines, but sending people will result in thousands of new technologies and disciplines, from medicines to zero-g flower arranging.
13) Because if we send anything less than all that we are to the stars then we are diminished.
14) Because Robots don't write poems or paint pictures.
15) Because I would not have liked to see Star Wars with just R2-D2 in it. ;>)
16) Because humans make robots look like what they really are, just souped up toasters.
17) Because I know of no robot that has actually expressed an interest about going into space.
18) Because I am human and I want to go.
19) Because it's NOT just about the science.