The change in temperature. Adding water is personal preference. I’ve drank with Scottish people who dump water in, convinced that if there was any burn whatsoever, the alcohol content was too high. Who am I to tell a scot that he doesn’t know how to drink scotch... that’s not to say we didn’t debate about it, though, but it was never a matter of right and wrong. Personally I add some when it seems necessary, which is rare. I understand the disruption of the o-h bonds of the long hydrocarbon chains, ultimately releasing previously trapped aromatics.... but making it cold.... I don’t get that.