For those right brain weirdo's, Newton's 3rd Law of Motion states that:

For every action, there is an equal but opposite and collinear reaction.

This carries over to relationships too. The rough translation would be (and this is not as precise as science since people are strange):

Most action(s) by one party is a direct/indirect result of earlier action(s) by the other party.

I say most because some people just do things that are unexplainable. But reasonable/intelligent people usually have reasons for their actions. Unfortunately, as life have it, most people have the memory of a Darmstadtium atom. So when they see an action taken by their partner, they forget way too easily to consider any potential causes for the bespoke action. They naturally assume that the action was without cause. That it was an action, and not a reaction. Sure the action and reaction never equals each other in magnitude and direction, meaning most times one or the other is more significant. But one should remember to always look into yourself whenever your partner seems to do strange things, before just assuming there was no cause. I did not say justifiable cause, however. But a cause does go a long way to explain the action...