French equivalent of the Japanese expression 百年の恋も冷める “… makes even a 100-year-old love turn cold”

Question

百年の恋も冷める

= (Something trivial) makes even a 100-year-old love turn cold/sour (in an instant).

In Japan, people commonly use this expression in a jocular and sarcastic manner to refer to the short-lived nature of most relationships that could come to an unexpected, abrupt end anytime instead of dwindling away over time, when, for instance, you happen to see your partner fly into an uncharacteristic rage over the most trivial of matters – in stark contrast to their usual demeanour – which you find off-putting to the extent that it would make even a 100-year-old love turn cold/sour in an instant..., jolting you from your lovely daydream back into cold reality.

This expression is strictly reserved for relationship contexts: The burning passion of love could all too easily die down at the sight of a small flaw that you wish you had remained blissfully unaware of. It might also be caused by a temporary lapse in the physical attractiveness of your partner. Anything seemingly trivial could turn out to be a trigger point!

Answer 1

Here are several suggestions:

Toutes les bonnes choses ont une fin.

Il n'est si bon cheval qui ne devienne rosse.

Il n’est si belle rose qui ne devienne gratte-cul.

Answer 2

Though not a phrase or a proverb, there is this common word: un tue-l'amour. It refers to a habit or a behaviour that will instantly put off your partner or your (potential) lover.

So you may have something similar to your idiom in those contexts:

Je déteste quand il met des chaussettes de tennis avec des sandales ouvertes. C'est tellement peu élégant que c'est un tue-l'amour.

Il ne se brosse jamais les dents. Cela devient un véritable tue-l'amour.

Answer 3

With your comment about
“[something] you might find yourself saying to yourself with a sigh of disappointment after having just caught your partner in an unattractive moment”,
in mind, ...
... I can perhaps imagine myself muttering (granted, after not 100, but 38 years of marital bliss):

[God, she doesn’t seem to realize that] there are limits to everything! ...

... as my much better half drones on about this or that while I’m engrossed in one of my favorite Soap Operas.

Although perhaps not fixed expressions/idioms, literal French translations of the bolded part of the above muttering might include:

“Il y a des limites à tout”
or
“Tout a des/ses limites” ...

... which, when muttered all alone, wouldn’t necessarily imply that whatever limits there are have been surpassed (or even, once surpassed, what the consequences might be), but in the right context, they would establish that even (especially?) old, enduring romances, or at least the partners therein, do have limits, especially to their patience.