Maybe because someone helped you into his coat, one arm at a time.
Or because once in his coat, you felt tiny and safe and warm.
Perhaps because you were standing face to face, looking up at him as he lifted the collar of the coat, his coat, to protect your neck from the cold.
And then, while still holding onto the corners of that collar, he leaned down to kiss you for the first time.
Standing under mistletoe is a wonderful reason to kiss.
Then again, so is someone helping you into their coat.