Plato wrote (and attributed to Socrates), "The unexamined life is not worth living."
You appear to be striving for that ideal in your effort to find reasons for every action you take. That is commendable. And as a former psychologist, I believe our actions do have causes, and that it is worthwhile to examine them.
Sometimes, however, the cause is little more than habit or self-expectation. We do what we are used to doing in a given situation. We act like we expect ourselves to act.
In other words, not every action is a conscious choice that can be traced back to a specific motive.