What is a good thesis for how Romeo and Juliet are in love with love and not with each other?
I like your idea very much, and I hope you go ahead and write about it. If I were writing an essay on that subject, I would feel obliged to try to show how Shakespeare himself wanted to convey that message. I would therefore have to look for proofs or clues in the text. If I could find such proofs or clues within the play, I would write a thesis that said something like this. There are many indications in Romeo and Juliet that the two young lovers were really more in love with love than they were with each other. Then I would devote one paragraph to each of three or four proofs or clues, with quotes, and conclude with a restatement of the thesis in slightly different words.
One indication, or proof, or clue, that Romeo is initially more in love with love than with Juliet is the quickness and completeness with which he forgets about the first girl he was so passionately in love with and switches his affections to Juliet. This comes early in the play and is a good indicator that Shakespeare was deliberately showing that Romeo just wanted to fall in love. It seems to me that their love is genuine even though both could have fallen in love with others if circumstances had been different.