Why do we find some things beautiful?

Short answer
Because they are normal

Long answer
In one experiment photos were taken of ten people and from these a cross-image of the ten was digitally composed. The eleven photos were shown to people who were asked which one was the most beautiful. Almost without exception was the eleventh picture chosen. We are probably programmed to appreciate the normal for reason of reproduction.

Question # 33 from “What is art” (must buy) by Ernst Billgren