Purity of tone. Density/thickness of sound. Strings throbbing with potential harmonies. Unexpected drop of one chord to another. Slow and dreamy long notes. Quick, energetic, foot-stomping urgency. Creative passing-off of leads. A crescendo and swelling of support instruments. A plaintive instrumental voice sounding through as support instruments back off.

There are a number of reasons. And there are many more than I've listed. :)