I like the symbolism of the rainbow's arch--it's going somewhere; it's not a straight line; it doesn't last forever--you've got to catch it while you can.
I like the math and science of it--the bending of light; the speed of light; relativity; wave-particle theories; refraction, diffraction, and the wonderful fact that even the least scientific among us can see it.
I like the fiction of rainbow promises--gold and leprechauns and mystery.
I like the promise of the Bible--the rainbow of Noah's ark, not magically created at the ending of the storm, but something God points to in the sky; something that becomes a promise of God that he'll never forget his love for us.
I like the splitting apart of things--me, an English American, a Catholic Protestant, a mathematician that can't add up and a writer that can't spell, a contradiction in search of a direction...
and I like their putting back together again into something that's greater than the whole; white light becomes a rainbow becomes a ribbon of white again.
Oh yeah, and I'm English. It rains in England. What English child could fail to fall in love with rainbows?