To continue with marriage ideas from laws of science:
Heat transfer naturally occurs from a region of high temperature to an adjoining region of lower temperature. That means when you put an ice-cube in a hot drink, heat flows from the drink into the ice-cube. It is possible to get heat to move from cooler to warmer, but it requires a good deal of energy to make it happen (a refrigerator or a heat pump).
The transfer of heat or love/affection/concern in a marriage works much the same way – it flows more easily from the one who has more to the one who has less. It will flow the other way, but it takes a lot of energy to make it happen.
Our feelings ebb and flow in marriage – this is natural. When you are feeling more loving than she is, there will be more flow from you to her than from her to you. When she is feeling more loving, it will be the other way. There will also be times when stress and business will reduce the ability of one or the other of you to be as loving as you would like – again causing the flow to be more one-sided. As long as the direction of the greater flow changes back and forth over time, this is not a problem – rather this is one of the ways we cover for the other, one of the ways that we function as a couple rather than two individuals. When it’s your time to be giving and doing more, don’t begrudge her. Realise that even what she is giving is probably costing her more energy than what you are giving is costing you. When it’s her turn to do more, don’t feel guilty.
UNLESS: Unless the flow is one way for a prolonged time, and not for a reasonable reason (such as long-term illness). If this goes on and on it will deplete the one who is giving. No matter how much we love, no matter how much we want to give, there is a limit before it starts to cost us significantly. If you have been doing more for a long time, you need to work to change that before it causes you to burn out. If you know she has been doing more for a long time, realise that it cannot and will not continue that way – she will burn out, and when she does she may have no desire – or even ability – to give again. If you suck up all her heat, her flame will go out – and then what?