Ihren schönsten Glücksmoment muss nicht immer das große Glück sein.
what does that mean?
The sentence isn't grammatically sound. It's probably supposed to be:
"Ihr schönster Glücksmoment muss nicht immer das große Glück sein"
Translating this literal is kinda hard. It means something like "Their/your(formal)/her most intense moment of happiness doesn't have to be an enduring effect."
Applicable to an affair, that doesn't work out or something like that.