- Meditations, 2:1
There are many things that men call beautiful. A painting, a certain landscape, a melody, a house or building, an attractive woman. By calling them beautiful we give them value. What is valuable is important to us, and we in turn think of them as "good" things. We naturally strive towards what is good, and posit our happiness and self-worth in their attainment and our misery and self-depreciation in their loss or our failed attainment of them.
The "beauties" listed above are transient. If we posit our happiness in them, our happiness will be equally transient. Perhaps worse than their impermanence is their emotive appeal. At best, they appeal to our senses (instead of our reasoning faculty), at worst they appeal to our baser desires and carnal instincts. Most of the time, they just tinker with our emotions, like the change from an upbeat tune to a sad love song.
The Stoic's happiness, by contrast, is secure, because his Goods are secure. He considers only the Good to be beautiful, and only Virtue to be good, and Virtue can always be his if he so chooses. His life can be truly, and permanently, beautiful.