Don’t let the simplicity of this question fool you. As with all things art, there isn’t a definitive answer, just experienced speculation.
Let’s try landscapes first. The world to our primitive ancestors must have been horizontal. The plains, pastures, oceans, rivers were wide expanses. Certainly trees are vertical, but the rest usually expands sideways. Think of an ancient man looking out on an open vista. As far as he can see, open space, perhaps a clump of trees in the distance. The only thing moving is the tall grass from a gentle breeze. Peaceful. Why? Because there was no immediate danger. As long as his belly was full that moment, there was nothing to fear in the distance. I think this restful feeling comes from our genetic make up. When we look at landscape paintings, they may call back these ancient feelings of peace.
Portraits are vertical because people (for the most part) are vertical. Our faces are vertical. We are vertical beings looking at a horizontal world. Portraits are less restful than landscapes in art. They are open-ended questions. When done well, they are a story about the person behind the portrait. There is a sense of mystery, who was that person, what was she thinking, why was she being painted. We use a great deal of our brain power to recognize faces, it is important to know who is a friend, acquaintance, or who is our foe. Again, these paintings are triggering something ancient in our DNA.
Our personal experiences are important determinants for what we like, what kind of art, what style of painting. But there is often something deeper, more primal tugging at our tastes. The ancient man staring at the quiet plain 10,000 generations ago, also painted exquisite animals on cave walls. Art is like that – modern and primitive at the same time.