Sherlock Holmes is so in love with John Watson that:

• He continues to talk to John when he is not there.
  
• Crashes John’s date with Sarah 
• Watches John walking to Sarah’s sadly from the window
  
  
• Thinks about John when talking about love- “I imagine John Watson thinks love is a mystery to me”
  
• Tells Irene not to compliment him, because John has already said every variant in the English language.
  
• Fakes his death, and spends years dismantling Moriarty’s network to ensure John is safe.
  
  
• Pulls John out of a bonfire, with no hesitation or concern for burning his hands
  
• Quickly hurries his parents out of the flat because he doesn’t want them to embarrass him in front of John
  
  
• Has a whole folder about John, and one of the pages is John’s face stuck onto a picture of the vitruvian man
• He is obsessed with finding out John’s middle name because he needs to know everything about him 
  
• Stops functioning when John tells him he is his best friend
  
• Is jealous of John’s ex-commander Major Sholto
  
  
• Tells a room full of people that John is the only thing he cares about- “There was one feature and only one feature of interest in this whole baffling case and quite frankly, it was the usual: John Watson”  
  
  
• “It’s always you, John Watson you keep me right!”
• Refers to the wedding as going “into battle”
• Yet still helped teach John how to dance, and writes the song for John and Mary’s first dance.
• Makes his first and final vow at the wedding to always be there for John and Mary.
  
• But leaves the wedding early because he can’t pretend to be happy for John any longer
  
  
• He turns to drugs to cope with the fact John got married
  
• Removes John’s chair because looking at it or seeing someone else sitting in it is too painful
  
• Fights to come back to life at the realisation that John is in danger
• Kills a man to ensure John is safe
• Rather than confess to John how he feels, he decides to tell a joke instead “Sherlock is actually a girl’s name”, to make John laugh one last time.
  
  
• On the plane Sherlock reads about his and John’s first meeting, this is what he wants to think about as he dies from the overdose.
• He creates an elaborate Victorian world in his mind where John and Mary’s marriage is falling apart because John has abandoned her for “a companion of dubious morals” (him) and the only way she can see John is by pretending to be a client.
  
• He imagines John saving him from Moriarty and declaring there is “always two of them”. 
  

Oh and he looks at John like this:

That’s the face of a man in love. John is Sherlock’s whole world, the one who makes him happy, the one he pines for and is miserable and depressed without. The one he will sacrifice himself for again and again because he will always put John’s safety and happiness before his own.