Holmes and Watson and why not the fantastic Mrs. Hudson are family, a family of their own choosing.
And I think that this vision is the one that comforts so many people that like these characters we have had to choose - to make our own family with people who are not bloodily related to us but who understand and accept us.
We like this dynamic so much because it is having a constant reminder that we are not alone that no matter that our biological families have rejected us for who we are (because of our gender identity, our sexuality, our disorders, being non-neurotypical, our way thinking and a BIG ETC. that can cause this division) or we have moved away from them because they hurt us, it is a reminder that taking that step does not imply that we will be condemned to be alone, we can find someone to connect with us , someone who appreciates us and supports us and loves us unconditionally.
It help us to don't forget that the kind of people who can bear you playing the violin horribly at 3:00 a.m. or doing experiments that explode or screaming in the middle of a nightmare and still give you a tender look.
The kind of people who can believe in you unconditionally, accompany you to do a crime if you only ask to do or they will do that to protect you and will leave everything to be by your side.
The kind of person who could believe in you when you don't, who listen to you and give you small and constant surprises to rejoice and that without a doubt they could sacrifice themselves to make sure you'll be fine.
That type of person exists they are real and they are in some corner of the world and knowing that is a great consolation.