Do you ever think about how In Hushed Whispers, we wake up in this dystopian future and our immediate goal is to use magic to find a way back to the world as we knew it? We came across companions who had lived and suffered in this timeline for a year; indeed, it was their reality. Yet while we could relate to them and have compassion for them, and seek to help and comfort them as best we could, when they died it wasn't heartbreaking because to us, they weren't real. To us, it was nothing more than a nightmare of a world that had manifested in the blink of an eye.
Do you ever think about how modern Thedas is no more real to Solas than the alternate, dystopian future was to us? He woke up after thousands of years to find an unrecognizable world filled with oppression and magical imbalance, where the elves he sought to liberate now inhabit the lowest position in society as either slaves, servants, or in poverty. It isn't simply that the world isn't valuable as it is. It's that the world does not feel real. The people in it are disconnected from the fade and so to him, they feel like tranquil. And yet despite this world seeming no more real than a bad dream, Solas still cares about the people in it. He always approves of taking time to help out strangers, and enjoys discussing with the other companions to learn their perspectives and even encourages them during particularly difficult times (such as Cassandra's loyalty crisis with the seekers or Iron Bull abandoning the Qun). He dislikes violence and decisions that take away peoples' freedom. Even though this world isn't fully real to him, he still feels compassion for the people living in it.
And if the inquisitor comes to see him as a true friend or, Maker forbid, falls in love with him, the illusion snaps. The present is no longer a nightmarish dream world. It is real, and the people in it are real. But... so is the world he came from. The memories of that time are still fresh, no more than a couple years in the past to him. They are closer to him than memories of the COVID pandemic are to us. He can surely still remember vividly the taste of foods long gone, the beauty of magic-imbued cities, and the vibrant life of a civilization that was never separated from the fade--which is the way the world was natural designed to be.
This beautiful world that has become lost to time, that exists only his memory, fell because of him. Only he has the power to fix what he broke. But how can he make that choice, when the once dystopian dreamworld becomes just as real as the world he feels duty-bound to restore?