that said, there are things that Duhigg calls keystone habits. I think a keystone habit can change everything else, kind of starts basic and then filters through your entire life.
I'll use an example. Someone who avoids dairy because the bros say "you shouldn't eat the milk of another animal." So you introduce a bit of cheese into your diet. That cascades through your life and choices you make are changed. You begin not believing them about, say, carbs...
In a relationship that has stagnated, often just kissing a lot and passionately can become the keystone habit that brings passion and interest back into the relationship.
So one thing can have profound effects in this way -- by being a catalyst that lets you change other stuff.