Those three little words come heavily-weighted...usually with care that has gone awry.
If you look closely, and dig deeply, you'll probably find that before those words were uttered, you did care. Very much. Perhaps, too much?
Quite often, so much energy is invested in whatever you may care deeply about, that it becomes a drain. Then, when things don't go the way you hope or expect, you become disappointed, frustrated or hurt. "I don't care!" can become a way to insulate or protect oneself.
If left un-transformed, these negative feelings and emotions can trigger the stress response.
What to do? Cultivate a more balanced perspective. Realize that you can only do so much. Activate sincere feelings of heart-propelled intelligence. Continue this practice as you run and march and stroll and linger through your days.