Our sages have synthesized the paradigms of creation into 39 categories. There are 39 categories of activities considered to be creative. These 39 categories of activity were necessary for the construction of the Tabernacle that G‑d commanded the Jews to build in the desert. All construction was to be suspended on Shabbat, despite the importance of building the Tabernacle. This serves as the source for determining which activities are considered "creation" and therefore prohibited on Shabbat.

Once it is clear to us that on Shabbat we abstain from creating and not necessarily from exertion, things make sense. When we drive a car, for example, we are creating fire (in the internal combustion engine). When we turn on the light, we are creating an electrical circuit. And so on with all other Shabbat prohibitions.

So, it's not an issue of stressful vs. unstressful—it's an issue of creative vs. non-creative.

Also, the six days of the week are days in which we transform the world outside of us. On Shabbat, however, we dedicate ourselves to our internal, personal, and spiritual world. In Kabbalistic terminology, the six days of the week belong to the "World of Speech," whereas Shabbat is the "world of thought." (One speaks in order to communicate with others. One thinks in order to communicate with oneself.)

Therefore, on Shabbat, we retreat from interaction with the tumultuous material world and turn our concentration to our personal spirituality.