It strikes me as a basic choice, something you say without giving it much thought. Simply justification to remain in the shade, stay away from the sun, preserve things as they are. Sometimes, though, it has nothing at all to do with the sun.
Other people step out in pursuit of light and warmth; there is a subdued consolation in holding back. Not all exposure feels good. Not every intensity is appealing. Some days, keeping covered seems safer than standing completely in it.
People believe the sun is always helpful. More of it implies more life, more energy, more freedom. And perhaps some people would benefit from that. For some, however, too much light creates pressure. like being observed more than you are prepared for. Like having no place to hide.
"Don't want a suntan" becomes more about control than about skin. Deciding how much of yourself you want to show. Choosing when to advance and when to back.
It is not exactly terror. Awareness is it. Knowing not every moment demands you be out there, fully exposed, attempting to match the brilliance around you.