If you’re veering into “insult comic” territory, it’s time to rein it in. Focus on sincerity, and if humor naturally slips in, that’s great. But lead with heartfelt words that remind them just how special they are.
Your special someone will never forget the first love letter they get from you. Like the first time you met, or the the first you kissed, it will be a milestone in your relationship, and a memento they'll keep forever.