“Personality” is a complex idea made up of emotions, thoughts, behaviors and more. Temperature is a direct felt experience — we know what it’s like to be hot or cold. In order to “get a hold of” that complex idea of “Personality”, we use the idea of heat and cold.
We say “he’s a bit of a cold person” to understand a person’s emotions, behavior, and temperament. This holds true whether he’s in antarctica or in the Sahara.
There are thousands of these conceptual metaphors (which give rise to thousands of expressions) and we will look at a few below.