After not very much thinking about it at all, John Watson suddenly realizes one day that you love the ones you kill for. He has to sit down to consider the implications of this, seeing as how only a day after meeting Sherlock, he killed a man for him, then lied to police to cover it up.

He probably should have taken that into consideration before shooting the man, perhaps even just taking a second to ponder how incredibly permanent and intimate it was, essentially linking you to that one person forever, especially when that person figured it out shortly afterwards, but it was sort of time sensitive.

He'd heard of it before, wasn't it some sort of saying, that you love the ones you kill for?
Or was it you kill for the ones you love?
Either way, John realized he was fucked.

In the weeks that came, John realizes a lot more things. He realizes that you love the people you carry home drunk or drugged.

And when their sort-of-boss films them on his phone, you make sure he doesn't get the part where he think he's a lion and start growling at other people to mark his territory. You and him laugh about it later, since it was hilarious, but doesn't belong on any website for other people to see.
John also realizes that night, that you love the ones you cancel your dates for. (She's angry, but it doesn't matter too much, since she breaks up with you by Christmas, because she realizes it before you do who you love.)
Just something else to add to the reasons of why John was seriously screwed up. Or perhaps just screwed. That was still under heated debate.

John also realizes you love the ones who love the ones you love, sometimes even more than you do.

Because when they arrive home to find that someone has harmed your landlady (not your housekeeper), they nearly murder the person responsible, only stopping at her insistence, although she is far too cheerful as she cleans up blood off the window afterwards.

You love the ones who hate the ones you hate, even if they don't have a good reason to. And vice versa. (Although honestly, Anderson, what the hell is with him? John has to struggle to not hate him, and sometimes it's just not worth the effort.)

John comes to realize, it's not just the big things.
You love the ones who drug you for experiments, and only mostly apologize. You love the ones who leave body parts in the fridge and use your favourite mug for an experiment. You love the ones who forget your birthday but can recall every detail of a mobile phone that wasn't his from years ago. You love the ones who get you shot, but hold their favourite scarf to your wound and look at you with desperation in their eyes (something you've never seen there before) and beg for you to stay awake. You love the ones who provide something you never knew you were missing, but now that it's there, you don't know how you lived without it. You love the ones who, despite being awful and annoying and insufferable, are amazing and fantastic and bloody genius, usually all in the same breath.

He still can't believe how blind he was to it all until it was too late.

Because finally, John realizes, you love the ones who die for you.
And that hurts the most.