The reciprocity of Solas and Lavellan
It's easy to read the Solavellan ending as Lavellan stepping in to save Solas and deliver him a happy ending. But the thing I really love about it is that to me, they save each other. Neither one could have a happy ending without the other.
Lavellan had an incredibly lonely journey in which she was elevated against her will into a religious figure of a religion she probably doesn't even believe in. Even when she tried to move past that, it kept calling her back. She may have disbanded the Inquisition, but eight years later when Thedas was in crisis everyone still looked to the Herald of Andraste to save them, and what choice did she have?
Even eight years on, everyone refers to her as 'The Inquisitor,' instead of her name.
Lavellan needs Solas because he's the only person who can really know what that feels like. He too was unwillingly deified, his myth swallowing the truth of who he really was. Ironically, only two years after their relationship ended, when she learns the truth about his past, does Lavellan come to know why he was able to see her and empathize with her very specific situation in a way that no one else did.
Lavellan is clearly a strong and dynamic person. She hasn't been moping all this time; she's moved on, made a life, no doubt achieved more amazing things in the intervening time. And yet - for eight years she's carried this piercing, bone-deep loneliness. Though she has close friends who love her, like maybe Dorian and Cassandra, they still can't really understand. The secret, painful longing in her to be witnessed; to be known.
The relief she must feel, after everything, when Solas finally lays down his plans. She doesn't have to be alone any longer. He's the one person who can still see her for what she really is, and he's finally ready to come back to her. It doesn't matter to her what he's done, because she knows his heart in a way that no one else does, just as he knows her in a way that no one else does. They speak elvhen to each other, echoing one another's cadence, because they are bound in a very deep way that no one else can really understand.
When she says 'We'll make this journey together, forever,' she's not simply offering herself to him. She's acknowledging that they need each other. 'There is no fate but the love we share' - because it is fate, in a way. They're incredibly lucky that they were brought together in this way, because no one else could have known them in the way they know each other. And I think that's one of the things that really draws people to their story - it's this beautiful dream about finding a person who truly understands and sees everything that you truly are.