Here is a decently sourced article:

http://laudatortemporisacti.blogspot.co.at/2004/10/wine-and-water.html

Generally they thought people would get really drunk if they drink it straight.

I don't think it is possible to answer why exactly did they think so but imagine this - if you would drink only wine and nothing else all the evening, every time you are thirsty, how much would you drink? A liter, two? That is really a lot. So probably you will have a glass of wine, two, then drink some water when you feel thirsty, then maybe another glass. Why not mix it? Wine mixed with sparkling water (making a spritzer) is popular in Germany, Austria, Hungary, partially because the amount enough to quench your thirst will got get you blackout drunk.

So it is conjecture but they could see drinking unmixed wine as ONLY drinking wine and nothing else for the whole evening, so using wine against thirst and no water at all. Now That would be a lot.

The article also refers Aristophanes: "A: Because the barbarians regard as real men only those who can eat and drink vast quantities."

So the idea of drinking unmixed wine would not be sipping on a small wine glass like today, it could be more like chugging pints of beer.

Or the quote of Atheneus: "gave wine to men for oblivion of sorrow; mix one of water to two of wine, pour them full from the brim down, let one cup jostle another" - to me it sounds like making a roughly beer-strength drink and handling it like beer - giving a good chug, not a sip.