Curves
Tip of the cap to Fast Company for this article entitled, “Why Our Brains Love Curvy Architecture.”
Ironically, the article doesn’t definitively answer the question put forth in the headline, although it offers some hypotheses. The writer does discuss a considerable body of research that suggests we find curving lines (in architecture or even product packaging) more aesthetically appealing than sharp, straight lines.
Also note that “more aesthetically pleasing” doesn’t necessarily mean “more functional,” although researchers are investigating that next.