Why did John always chose the weird ones to be friends with?

First is was the girl with
anger issues and ADHD in primary school. He connected with her, he thinks,
because she reminded him of Harry. They drifted apart when they went to
different secondary schools. He still kept in touch with her, she is now a
police woman up in Manchester: no one wants to mess with
her!

Secondary school, John
was the outcast. Huddled in the corner of the classroom, only coming out of his
shell if someone insulted him or his sister. Unsurprisingly, he was given
detentions on a regular basis for fights and broken
noses.

The cross dresser. Ah.
Yes. That was during university. Tony used to boast that he was going to be the
first openly cross dressing doctor in London. Unfortunately for John, as they
were room-sharing in halls, everyone thought they were, well, together. He is
doing well. Tony is apparently working for celebrities and making a great deal
doing so! All his old friends are doing so well, and John is exactly the same as
he always was, maybe with a bit more added excitement from his current
friend.

"John! John! What do
you think? Isn't this all so exciting!" He sighed. Speaking of weird friends...
Looking up, the countenance of Sherlock Holmes: genius, lunatic, consulting
detective, bouncing up and down like a five year old with the key to a sweet
shop. Only there was no sweet shop. Oh no. Sherlock had just been given access
to a serial killing.

Why did
John always chose the weird ones to be friends with?