Why?

Arwen didn't know why she asked that suddenly, but it was definitely something she had wanted to know all this while. Previously, she was scared of getting disappointed if he said he wasn't the one. But today …she felt it no longer mattered. 

Even if he weren't the one that night, he would still be her husband —the one she could no longer think of parting with. 

When Arwen didn't hear him answer, she continued, "A month and a half back, I met with an accident at Palace Road in the East. I would have died that night if someone hadn't appeared at the right time, like today, to save me. I was in a delirious state and had no visual remembrance of the man, but I still feel indebted to him. He saved me when someone abandoned me to die. I don't know why but I feel a strange kind of longing for him, like I had known him from the past."