Contrary to what people think, good design is explainable, though designers may do most of the creation in their head. It's like asking why some code should be written this way, there's always a good explanation for good code.

Good design comes from a process, UIUX design for example is a result of product design, which is a result of user needs analysis and requirements gathering in the form of understanding a problem domain deep enough to know what is wrong with the current design, or what is expected of the functionality. This is not about vague "design thinking", but actual auditable design processes that lead to a good product.