What makes a word beautiful?
Is it the sound of the word or its meaning?
The study of the pleasantness or unpleasantness of the way words and phrases sound (regardless of their meaning) is called phonaesthetics.
But I think the combination of sound and meaning is what makes for pretty words.
Consider the word “diarrhea” which trills off the tongue but isn’t a pleasant topic to ponder.
But the word “diary” is equally pleasing to the ear and has an intriguing meaning.