The Ancient Greeks almost always diluted their wine. They also lambasted anyone who didn't, such as the Gauls and the Macedonians. Ancient Romans also frequently diluted their wine, but didn't seem to be nearly as disparaging of the drinkers of undiluted wine.
Nowadays, in almost every culture that permits the drinking of wine, it is considered improper to dilute it. If I were to go drinking with friends and dilute my wine, I would get strange looks. Also, when I visited Greece and Italy (in 2016 and 2018, respectively), the people of those regions nowadays drink their wine undiluted.
When, and how, did drinking diluted wine, change from being the "right way" to being the "wrong way" to drink wine?
Part 2 of 2: Why did Coffee appear to have been discovered so late?
While waiting for an answer about why wine was diluted, This older answer by u/Qweniden gives a social reason certain cultures diluted wine.
While on a tangent but often mentioned on questions bout alcoholic drinks in the past, is an answer by /u/sunagainstgold about safe potable drinking water being available in medieval Western Europe due to water works and irrigation but was frowned upon socially because it was free.
I'm sorry and hope this doesn't get removed but just to clarify-- isn't all wine diluted? Doesn't diluted alcohol just mean it's not solely alcohol and obviously nowadays nobody drinks pure alcohol and seldom have people or groups done so if I understand correctly. What does diluted wine mean in this context?