What is it that causes the human person to recognize beauty?
With every other creature on this planet, nature drew a lower and an upper limit. A lion can kill any creature in the jungle, but hard as it tries, cannot fly. More importantly it does not even want to fly. If a lion satisfies its hunger it is content.
Humans are unique in that nature did not specify an upper limit. Once we eat, a hundred other problems crop up. No matter who you are, every human dreams of being something more that what he/she is right now. Why does this longing exist? To understand this you need to understand the question "Why are you asking why the desire exists?".
This longing is infinite. To fulfill it even for a moment is to experience beauty. It cannot be put into words. Words are the domain of thought. And thought, no matter how vast, is physical and hence finite. If you could describe beauty in words, a person could read about the apollo of belvedere and be mesmerized by it.
To put it succinctly, there is reality outside you and reality as is projected in the firmament of your mind. If you get to experience reality as is you find it beautiful. To experience reality as is, you need to halt the distortions of your perceptions. When you find a sunrise beautiful, you don't want to ask why, you don't want to sleep with it, you don't want to eat it. You want to be in it. In other words, you are it. In that moment, when what you want and what is unite, you realize it as beauty.
An interesting conversation that follows the concept of beauty.