John doesn't have any other friends

 (Sherlock meta by Loudest Subtext In Television)

Question: 

I don't know if this is really your area (your division?) but have you given any thought to why John has like no other friends? Like aren't bachelor parties or stag nights or whatever supposed to have more than two people? People like John, right? I know Sherlock is his BFF but didn't he have drinks with the guys on some nights when they lived together? He has to have some outlet.

Loudest Subtext In Television:

People don’t like John, actually. From the very first episode it’s hammered home that he doesn’t make friends easily — Mycroft outright says it when he’s going through John’s therapist’s notes, and John can’t even think of anyone else to room with, and we get constant shots that emphasize how alone John is — and he doesn’t particularly like hanging out with the friends he does have. John sometimes goes out to have drinks and it gets mentioned on his blog, but not that often, and more often than not we see him dodging invitations in the comments or not even responding.

John is incredibly rude to people — rewatch ASiP and see how he treats Mrs. Hudson, and what a trial it is for him to have a conversation with Mike Stamford — but he’s not as outright malicious as Sherlock so we tend to forget John is usually a jerk. John knows to take care and be polite with women he wants to go out with, and people who are suffering, but that’s about it — and even then not always, as we see at the beginning of His Last Vow. His default mode is rather surly and terse. He’s patient compared to Sherlock, and impatient compared to everyone else. He rarely smiles at people, and most of his genuine smiles are reserved for Sherlock — especially when Sherlock is being awful or makes some off-color joke.

The difference between John and Sherlock is that John knows when to bottle up the rude stuff he wants to say. He even says in the Many Happy Returns entry that he views Sherlock that way: “He was everything a good person should be. He’d just often say what he was thinking rather than lying to protect our feelings. Maybe we should all be more like that? Maybe we should all be more honest?” John seriously thinks everyone has thoughts as rude as the ones Sherlock expresses and John bottles up. That says a lot about just how “nice” a person John is.

Plus John is a straight-up instigator. He’s constantly looking for an excuse to kick someone’s ass. It’s all over his blog and the show. There should be videos of John on Worldstar with him just being held back and yelling, “I WISH YOU WOULD!  I WISH YOU WOULD!” He almost fought that guy during the drunken deduction scene in The Sign of Three. How much fun do you imagine it is to go drinking with John? You always have to be worried he’s going to do something like that.

Sherlock wasn’t mistaken in Many Happy Returns when he said John’s friends don’t like him. You don’t have a ton of friends you get along with and end up having Sherlock Holmes for a flatmate. You have Sherlock Holmes for a flatmate because no one else will have you. John has a gentle caretaker/protector side and it’s touching, but most people never see it.

This is why they bond so tightly, other than that instantaneous chemical attraction business.

They’re both very lonely and don’t know how to reach out to anyone. They fun each other and realize they don’t have to reach out. They can just BE. And that’s pretty important for these two, because they’re pretty much emotional disasters.