I have been using something like this...
The barrel is made up of staves that represent various things that make us feel loved/in love. The water in the barrel is that set of feelings. The problem is that certain things might be really strong, that is, at maximum, while others might be less than the best possible. The barrel can never hold more than the lowest variable, the minimum factor.
Now for us, we might easily provide one of the variables without much effort. Our spouse, however, might not see that as a problem and in fact, that factor might be one of those on which we would score very high. The thing is, when our spouse complains or otherwise states that something is lacking, it is this set of easy to provide things we supplement first. We always seem surprised by the fact that we gave more, did more, provided more and the barrel never got any fuller than it began.
Think love languages here... If what I do to show care is something my wife needs very little of, or little of right now, no matter how much beyond what I have already given I continue to give, her feelings toward me never grow stronger. Instead, she tends to voice her dissatisfaction over the state of our relationship, which to me comes across as nagging... She feels neglected. She feels less fulfilled. She feels the love running out and no matter how much I try to put in, the level never goes up...
Until, that is, I find a way to improve the weakest piece of the whole puzzle. Once I improve that one single part, the level increases. It goes as high as I can build that piece until some other piece is now the one that is the lowest, the minimum factor.
So if we can find a way to identify and communicate to each other what specifically it is that will increase our love toward one another, and once we have improved that one thing, we identify the next thing that is keeping us from filling the barrel and work on it... We can continue around the barrel, identifying what is most lacking, what is the minimum factor and improving it until it is no longer the weakest link. We then move on and fix the next weakest and so on until the barrel is easily filled and kept full without a lot of changes, jumping through hoops or self sacrifice.
Love bank...Love bucket... Love Barrel...
Spend the time and energy fixing what will make a difference and stop wasting energy on changing stuff that is either good enough already or that tweaking further can actually cause to end up broken because we suddenly starting doing it wrong after years of doing it right.
In Harley parlance, the barrel is made up of various ENs. Some need improvement. Others are fully adequate. Some might be good for now, but once others start to improve, those too will need to get better for a couple to reach the goal of being in love again. A single problem with one EN that is highly needed going unmet, allows the barrel to drain almost completely.
Love busters then, are holes we shoot in the sides of the barrel. They don't keep us from filling the barrel beyond that level, but it doesn't stay filled once we stop or slow our effort. The relationship becomes that black hole that we can never fill. He/she is never satisfied, never getting enough, never staying happy with us for long.