When Solas awoke from Uthenera, he was shocked to find how cut off people were (even mages) from the Fade so that it was akin to "a world of Tranquil" to how things had been before, while the Elves were diminished both in magical ability and had lost their longevity.
He has been trying to divorce himself from the notion that anyone in Thedas are "real", leading him to execute Felassan at the end of Masked Empire for both defying his orders and coming to see the modern Elves (or at least Briala) as worth saving.
Solas finding himself enamoured with Lavellan (especially a non-Mage) starts to make him realise what Felassan was getting at. It leads him to start to question himself, as it will be hard to carry out his plan if he sees everyone (especially modern Elves) as people, rather than a mistake that he needs to correct. Solas' internal conflict according to Cole is;
"You're real and it means everyone could be real. It changes everything, but it can't!"