He doesn't remember much about the day he met her, but he does remember when he started to fall in love with her. He remembers it starting at that Christmas party and maybe it had something to do with that dress she wore, the one she's wearing now, but after that he remembers going out for drinks and at that god-awful point in his life he felt like being with her was the highlight of it.
He's always loved her spirit, her intellect and her kindness, but most of all he's always loved her courage. She doesn't know this, but she's brave, braver than he'll ever be and before she asks him how he knows that, he knows it because he's seen it. She's saved his life and he's seen how brilliant she is, how she handles people like Moriarty and how she cares for Hope.
He sees so much of her in their daughter. She keeps getting stronger every day. As small and frail as she is, she keeps on fighting just like her mother does.
He could never do that. He may act strong, but he'll never be as strong as she is, but he wants to be. She's inspired him to be a better and stronger person and he's been happier because of it.
It's only been a year and it's been a rough one to boot, but as long as they're together they can make it through even rougher times, just like they always have and always will. It's why he wants this year to last the rest of their lives.
When he tells her all this she's in tears, knowing exactly what he's about to do and to ask her.
He goes down on one knee.
"Molly Hooper," he says. "Will you marry me?"
"Yes!"