It's a shotgun wedding!

Yes yes Holmes uses his talent for deduction to catch criminals but have you considered: the gifts so carefully considered they barely register as gifts. Watson, who has no family and few friends, doesn’t celebrate his birthday, hardly remembers it himself, and has certainly never told Holmes the date. And yet every year on that day they end up in a pleasant part of the country on business, with a little spare time to fish or stroll through the woods or eat a particularly good meal at the local inn. Every year Holmes gives Watson a small, oblique word of praise that sits in his mind like a warm ember. And every year Watson falls asleep thinking one year older, and it was quite a nice day.