At the time I had no satisfactory response for him and replied to his query by saying something like, "Well, I guess that's true, but I still like it even if it isn't useful. No one is forcing you to care about Jupiter, but I think it's beautiful.". It is certainly true that being wowed at the view through someone's little 8 inch telescope isn't going to help my finances, and only just getting started with astronomy at my age, it's highly unlikely that I'll progress far enough in the field to make a meaningful contribution to science.
But there's so much more to it than that!
I don't have to be a scientist to find relevance in experiencing what I can of the universe. Even if I never make a great astronomical discovery, even if I do nothing more than marvel at the lovely sights scattered across the night sky (or here on Earth for that matter), even if I receive no direct tangible benefit from being able to recognize and identify a star or planet, it still does me good to participate in the great tradition of human curiosity and imagination that has so far distinguished our species from others on this planet.
While we hurry around navigating highways packed with fellow commuters, chat on our cell phones (ideally not at the same time!...advice I need to heed more frequently as well), swipe our credit cards, email our friends, and participate in the minutia of our daily regimens, we almost never consider the origins of all this convenience. As light from our growing cities gradually washes the stars from our view we often forget that science and its resulting technological and intellectual benefits to the human species began when our distant ancestors looked up into the heavens, marveled at their strange beauty, and asked "why?" As we go about our busy lives, it's certainly easy to forget the fundamental and continuing importance of an action as simple as walking outside and recognizing a planet or familiar grouping of stars.
Johannes Kepler wrote in his Mysterium Cosmographicum of 1596 (translation quoted by Carl Sagan in "Cosmos"):
"We do not ask for what useful purpose the birds do sing, for song is their pleasure since they were created for singing.
Similarly, we ought not to ask why the human mind troubles to fathom the secrets of the heavens...the diversity of the phenomena of nature is so great and the treasures hidden in the heavens so rich, precisely in order that the human mind shall never be lacking in fresh nourishment."
When astronomers and mathematicians first effectively proved that the Earth was not the center of the universe it was considered heresy in a society that largely believed the universe existed entirely for the benefit of humans (as is poetically suggested in the above quote). Continuing scientific discoveries seem in once sense to make our place in the grand scheme of the universe less and less central. We're tiny creatures on a tiny speck of dust orbiting a relatively tiny orb of glowing gas situated rather inconspicuously in a remote arm of the Milky Way galaxy...itself a fairly typical collection of stars, gas, and dust only one among billions.
To continue this pattern, we could one day discover that we are only one of countless other populations inhabiting the universe (which by some estimates is quite probable). Ideally this information should not frighten, anger, or depress, but rather should serve to fully unite our species and allow us to rise above our trivial, and newly "local" disagreements as we venture out into a vast cosmic community. It's also possible (if arguably unlikely) that we could find ourselves utterly alone in the universe. If this is true, it should also impress humanity into resolving differences if only to preserve our species and therefore the universe's sole expression of life.
The real danger to us all would be a collective forgetting of our inherent curiosity about the world and a dampening of our communal thirst for discovery and enlightenment in favor of a careless and superficial existence. As knowledge progresses, truth has the ability to liberate society beyond the prejudices and petty conflicts brought about by our own fear and ignorance. If we instead allow our baser impulses to drive our destiny, the use of technology originally conceived through the methods of science could also be used to wipe out life on Earth as we know it. It is a delicate balance and one I'm certainly unable to properly address in detail.
In any case, I'm glad I still get a rush from looking at the stars. And it makes me happy to think there are still lots of others who do as well. This, to me, is hope.