This isn't actually a proverb, but a proverb-like saying found on a building in lovely Ascoli Piceno in north-central Italy (see below), in the Marche region of Italy. Ascoli is well worth visiting. While there, don't miss the local specialty of olive ascolane, scrumptious local stuffed olives.
As I was saying before my attention was waylaid by considerations of food (happens all the time). In Ascoli they have a number of sayings sculpted on the front of buildings. Here is a pithy statement from 1529 on why life is not all it could be:
Chi po non vo/chi vo non po/chi sa non fa/chi fa non sa/et cosi' il mondo mal va.
In modern Italian:
Chi puo' non vuole/chi vuole non puo'/chi sa non fa/chi fa non sa/e cosi' il mondo male va.
In English:
He who can doesn't want to/he who wants to can't/he who knows doesn't do/he who does doesn't know/ and this is how the world goes badly.
This is actually pretty deep when you think about it. And on this cheerful note, I wish you a happy Fourth.