“If it’s not broken, don’t fix it” means that if something is working sufficiently well, leave it alone. The reasoning behind this is that interfering might cause problems, and at the very least you’ll waste time doing something that doesn’t need doing.
We need reminding of this because humans have a natural inclination to improve things, to meddle, to leave their mark.
Sound advice, certainly. But no advice is appropriate to every situation. Sometimes things can be improved with a little tweaking, and routine maintenance is often overlooked.
Neglect is a common feature of many relationships, which can often be improved if they are given the right attention.