I’ve observed that the amount of energy one can put into a relationship (or anything else) fluctuates over time. As any physicist can tell you, adding two waves whose amplitude is 50% will only get you to 100% if the two waves are perfectly in phase. People who are perfectly in phase with their partners don’t need relationship advice, so let’s assume we’re not talking to them.
If the rest of us want two oscillating systems to add up to 100% output over time, even though their frequencies and phases are different, then each of them needs to put in 71%.
Caveat: my wife has been rolling her eyes at this calculation for 20 years.