You'll notice that characters created around the same time as each other tend to share traits that are popular at the time or big with the artist/director/etc who works with them.
However, I'm not here to talk about personalities. Those have already been covered by many interesting authors who explore the topics far more effectively than I could. What I want to talk about is common overarching themes I have noticed in each generation of princesses.
A common theme with the first three Disney princesses (Snow White, Cinderella, and Aurora) I think is maintaining hope and optimism...