Actually when he starts falling in love with her! (OMG THIS ALMOST DIDNT WORK ITS MASSIVE)
Toph was his best friend. Of course he’d always loved her and thought she was beautiful. But somehow, spending the day with her and Lin, seeing her so openly caring and engaging with her daughter, seeing the unguarded happiness in her eyes at his return home, somehow… Sokka started to cross that line and fall in love.