Yes Jareth used to be I love with Sarah.
Now if you have seen the movie and this theory keeps coming up I bet the first thing you'll say is interesting but bullshit.
We fangirls want a REALLY romantic plot line.
But to me in this theory in general reminds me of The Hunted Mansion. When the Ghoul gets triggered by the fact that the mom looked exactly like his lover... but let's keep going in the theory.
Sarah wasn't the first girl to get in the Labyrinth. There was another girl named Sarah.
The first FIRST Sarah was in love with Jareth but then she died and Jareth decided to look for the next Sarah.
He saw that there was a Sarah that resembled his lover. So he makes Sarah #2 ask him to take Toby away. So he did blah blah blah.
But!
This was a way on tricking her in the Labyrinth and stay with him forever. By the time he gives triggers a memory of him with the first Sarah and gives it to the second Sarah.
(Remember that he's deeply in love with Sarah.)
He makes remember but since the second Sarah never met him she decided stop go for Toby.
As she passed through the Goblin city and entered the castle to find Toby Jareth sings directly to her. (Within you)
He gets sick of living through her expectations and he can be cruel but also has a loving and sweet side.
The second Sarah really never understood because the just meet 13 hrs ago. So she goes directly to save Toby.
Since Jareth has a deep love for Sarah he let her go to her own universe and he looks for another Sarah.
Lol so yeah that's pretty much one the bullshitty theories I didn't like. Idk is just a bit... weird. To me.