Basically, the Princess had been having prophetic visions as a kid, and they'd always be about the person who'd help solve some of the problems going on in the kingdom and the surrounding areas. In this case, that's Tooru. She'd been having those visions of him appearing for years and fell in love with him to the point that, without needing to use a slavery contract, she willingly got into bed with Tooru. It was to the point that Tooru got a level up for getting her to devote her heart to him since she has a really rare class that grants prophetic visions in addition to being able to do it without a slavery contract.