She's never been in love before, but she knows how it's supposed to go. It's easy, by degrees.
She would compare it to the melody of a song, but it felt more inevitable than that - the harmony part, perhaps. The rhythym of their lives was determined long ago, and whatever melody they wished they could have put on top of it. Little things, the places they went, the kisses - those were the melody. Falling in love was the harmony, because it was not destiny, but it was still right and natural and all the little things are imporved for it. Of course she thinks in musical metaphors. Her life is music. Madoka has lived to the sounds of a violin.
She does not know what metaphors Shido would use to describe their love. It is more probable that he would not use metaphors; Shido is a man of practical simplicity, straightforward about almost all things. (The exceptions are important, however, and she notes them, remembers them, tries to draw him out.) Metaphors would only get in his way.
She thinks of people in musical terms. People have their preferred keys and measures; their lives are variations on a theme, measured by the rhythym of their days.
Although Madoka's specialty is not composing, she has turned her hand to it a few times. Most recently, she tried to compose something for Shido, about Shido. It did not work. She thinks of people like pieces of music, composes themes and hums them to remind herself of the faces of people she'll never see, but she never came up with Shido's theme.
He has never fallen in love before, and he did not know how it was supposed to go. He didn't have to ask. Things went as they would, and Shido went with them. He was not sorry that it was Madoka.
He does now know what it was that drew them together, but he knows that he will stay with her. It is not a result of loyalty or devotion or desire, although a casual observer might conclude as much. It is simply a fact, an immutable aspect of himself. Fuyuki Shido has all the beasts of the world at his command, and he is a strong man and an honest one, and he will stay with Madoka forever.