Alcohol is one of the issues that each kohen had to learn to handle.
Like every other Jew he recognised the truth of the Psalmist’s words, Veyayin yesammach l’vav enosh – “wine gladdens the heart of man” (Psalm 104:15). This principle led to every Jewish celebration entailing a cup of wine – Shabbat, festivals, b’rit milah, marriage, etc.
Each day was special, and the wine enhanced the joy. But all in moderation – no going overboard with the wine, no going overboard with the excitement.
Result: drunkenness was always rare amongst Jews, and Jews were known for their clear heads.
Some groups, notably the kohanim, had to be particularly careful.
There is a Talmudic saying, Nichnas yayin, yatzah sod – “when wine comes in, discretion departs” (Eruv. 45b). In one sense it means, when applied to the priests, that because of alcohol one can become too clumsy and careless to perform the Temple ritual.
But sod also means a secret: in particular, one of the Divine mysteries. A drunken priest cannot think straight. He loses his instinctive understanding of the inner meanings of the words of worship and the ritual actions that are his sacred task.
When the priest cannot minister properly, the whole people are spiritually diminished.