Sif loves Thor the way she loves to fight. With a passion that need never be questioned because it was never a query at all. It was something that was and is and always shall be.
She has never thought about why she feels this fierce loyalty burning hot in her lungs and heart the way wounds burn and bleed after that first slice; full and flowing, hot and quick; but lasting in its ramifications.
Sif loves Thor not because he is her king and that is her duty, but simply because that is what she was meant to do.
Sif is a shield maiden, she serves and protects the golden haired man whose pride extended far enough to reach her and cast light into her existence of constant carnage and fight.
She loves him because to him it is not 'the Lady Sif and the Warriors Three', it is simply 'Dear Friends'.
She loves him in the same manner with which he does everything; easily and without a second thought.