Anyway when Garak says “That’s the thing about love. No one really understands it, do they?” He’s talking about Julian.
They set up this parallel between Quark’s struggle to let Natima go and Garak’s struggle with his conscience whether to kill her and her students. Then they equate Quark’s love of Natima with Garak’s love of Cardassia. Which totally fits, and I love that actually!
But! Also! *Julian* was literally the one who told Garak, “Before you can be loyal to another, you must be loyal to yourself.” There’s no way that wasn’t playing in his mind in that final scene. I don’t want to diminish Garak’s own morality and his desire for a free Cardassia as his chief motivations, but I think Julian’s words, and Garak’s developing feelings for him, helped tip the scales. When you’re trying to make decisions for yourself that you want to make, it helps immensely to know someone you care about agrees with you.
Also Garak understands his love for Cardassia. It’s an intrinsic part of him. But these feelings for Julian? For this idealistic human doctor? That love is the mystery.