A love letter is moving when it could only have been written to one person. The details do that work: the first thing you noticed, the habit you love, the ordinary Tuesday you still think about.
A moment only you two share.
How you met, a trip, a hard season you came through, a joke that is still running.
What you notice.
The small things: how they sing while cooking, how they look after their friends.
What they have changed.
Say how your life is different because they are in it.
What you hope for.
Close by looking forward: the next year, the next move, the rest of it.