Recently I thought about this legend and asked myself: if there is no dragon in real life, what does the dragon represent?!
I think princes rescue princesses from their loneliness, cuz when your prince appears, you’re not lonely anymore.. family is important (for sure), but every member usually has his/her princess or prince, and they can’t give you time they spend with their lovers. 
So till the moment prince comes to save princess she’s stayed completely alone in an empty castle. (and I think everyone fears it the most: to have no one who loves you and stay alone till the end of your life)
But if we make the legend like this, it’s gonna sound stupid, isn’t it?! Like why princess wouldn’t go out and find someone?! So, loneliness should be materialized, be something terrified and huge to show the value of a prince! And, imho, such creature as a dragon plays a good deal.
Therefore my opinion is that this legend is about loneliness and the triumph and relief of killing it!
*This is my thoughts only