I think that the rainbows resonate deep inside us, striking at some visceral sense of wonder and awe. I would say it was their unexpected and ephemeral beauty, but I've felt the same wonder from very different sources. There was the majestic grandeur of the Grand Canyon, which you can see a thousand times in pictures and on television, but still not truly appreciate until you stand at its edge. In Rome and in Egypt, I was moved beyond words at walking on the same stones where the ancients had walked millennia ago. There's nothing comparable to holding an infant's tiny perfect hand on my palm, or watching him sleep on my chest.
For me, the sense of wonder is profoundly linked to my faith, and my belief in something greater than me, than all of us. I can't personally imagine one without the other, or indeed of doing without either at all. Does the absence of faith diminish wonder? Or does it increase it, having no explanation? Is wonder universal, or are there people who do not feel it? Are some people more attuned to wonder than others? We can test the basic senses, but how could we possibly test this?