“Spiritual” is a rather loaded word to use in this case, implying as it does the existence of a spirit. Nevertheless, I’ll use it for the nonce.
When I paint, I invoke a degree of ritual and symbolism to put many religious ceremonies to shame. I never just throw the canvas on the easel and start. I could do that (I can buy pre-made, pre-gessoed canvasses), but that’s simply not the proper way to do it; not “proper” in the sense that buying pre-made stuff is wrong, but it subtracts an important element from the experience. There is a fairly rigorous ritual the night before, and a fairly rigorous ritual in the morning. Then of course, there’s extensive symbolism involved in the painting itself, and it’s not an arid symbolism; I use the symbols I do not because, say, historically we have believed skull==death and I want to evoke death, but because the skull actually invokes the reaction in me, so it is necessary to use it to achieve the reaction in me that I want. Painting is a symbol-laden ritual, and what it actually intends to achieve is not just a good painting, but the exploration and working through and reinforcing of certain emotions and a way to bring out in the open and deal with important issues in my life. Of course, all these things must result in a painting that is also aesthetically pleasing. So we have ritual, symbolism, introspection, aesthetics…why the hell would I need to go to church?
What does a “spritual response” say about humans?
We like looking at things that aesthetically please us, I suspect originally due to an association with health and plenty and safety. We can train our perceptions to find many other things aesthetically pleasing. I didn’t find modern art pleasing until I learned a great deal about it and saw many works; then I found more modern pieces to evoke the “spiritual” rush that I had previously only found with more realistic art. And in addition, many of the more realistic works I loved before then seemed banal and bland.
We like ritual. It makes us comfortable, and reminds us of the times when we performed the ritual before. Repetitive actions, if performed meditatively, can center us and prepare our mind for introspection. Fairly accurately repeating a beneficial action is a very useful trait and frees us from having to constantly flail about wondering what to do next, so doing so gives us a feeling of contentment and security.
Spending time to step back from the world and think about your life, decide what you want changed and what you want reinforced is generally good for us. Being able to be completely absorbed in a process of thought is useful as well.
Our deep-seated love of symbolism comes from our ability to think in metaphor; hell, I’d be pretty impressed with myself if I could convey to other people that paint-on-cave is a horse. It’s almost magic. I mean, you can take one thing and have it be something entirely different, and the excercise of that ability is amazing. It can make you believe the two separate things are actually somehow connected. This is, I think, how symbolic magic came about; and why plays, music and art, which are only symbols of “the real thing”, evoke such a powerful response in us. This is why we have our symbolic superstitions: a “winding sheet” on a candle signals death, break a mirror you lose your soul since you shattered an image of yourself, and so on.
Going to church or viewing a play also reinforces a sense of community, too, which makes our herd-instincts happy.
Since things like understanding metaphor and analogy and fondness for ritual and taking the time to actually think without distraction about something are good and useful attributes, we tend to have those capabilities in spades and things that incorporate these preferences “fulfill us spiritually”. We have umpteen thousand years reinforcing these behaviors, and they’re generally beneficial. Take then to extremes and you get OCD and belief in sympathetic magic and monomaniacs, but since these behaviors in smaller doses are generally so useful to us, our love of the less-necessary aspects of art and ritual does us no real harm and training those capabilities can do a great deal of good. Stretch your proficiency at metaphor by studying art, and perhaps you can use that to visualize the workings of a new scientific theory. Meditate and learn how to concentrate better. Frequently go to church and learn how to interact within a community.
That’s my opinion as to why people get a “spiritual” reponse from church or art or music, anyhow.