AN: The smarmy things my brain comes up with late at night. Un-edited fluff for your viewing pleasure.

Standard disclaimers still apply.

Sherlock lives in possibilities – in the "what ifs" of life. They fly around in his head, asking, "what if the brother hid the murder weapon on the roof?" and "what if he said this to Donovan, what would happen then?" and it goes on and on through the possibilities. He weeds them out, lightening fast, discarding the ifs that result in someone punching him and the ones where the facts don't match up and the ones that let the criminal get away. He wouldn't be able to live without them – couldn't survive without the choices constantly running through his head. He lives in a constant state of uncertainty – not that he doesn't know what is going on, but that he doesn't know what he will choose next.

Sometimes he runs through different scenarios – different concepts of himself – to figure out how he really feels. What if he liked Anderson – well, logically, a positive emotional response to Anderson would lead to ease of useful social interaction, high recall of information from him, etc. None apply – therefore, false statement, next scenario. Next world. Next emotion.

Sometimes he creates what ifs for fun. What if he was in love with Lestrade? That one entertained him for days. Any strong emotional attachment to the good detective would stem from either solving crimes together or his help with the drug issue – well, not anymore, that was… better now. But a romantic relationship involving Lestrade would be quiet, caretaking-based or shared-passion-based, and given the DI's predisposition to solid, plodding police-work and linear thinking, it would be long-term. He asks himself how it would start, end, look like, feel like, figures out the reactions of everyone remotely related to the two of them throughout and so –

Sherlock imagines himself in such a relationship, and then goes back to work. It doesn't fit reality, and can be discarded.

Other what ifs are more useful: what if he dies tomorrow? He knows it's more than an idle question. Dies tomorrow – papers to Mycroft, personal effects to John, relevant case notes to Lestrade. Mycroft explains it to Mummy, Sarah takes care of John and Mrs. Hudson. Lestrade carries on, hopefully able to solve the case that killed Sherlock with the help of John and everything else he left behind. Considering his current case, what about John? Relevant risk factors, probability of Moriarty involvement, John's current temperament – text, call, bring along, or do not inform of absence. Don't choose last, not in a death situation.
Always at least text now.

It's not surprising that he applies the same principle to John. He's the natural subject for Sherlock's experimental multiverse. For his unique system of figuring out feelings. So his brain asks the natural next question in the spectrum (Coexistence with this person possible? positive or negative emotional reaction? Check extremes. What if he hated him? False. Then -).

"What if he loved John Watson?" And he follows through with idea, floats the idea of that world through his mind. For a while, it's just an intriguing possibility. But it sticks in a way the rest of them didn't, and he can't stop without an answer. If he were in love with him, then he would be jealous right now. Check the hypothesis: was he jealous? Hm. Need more data. If he were in love with him, then the next thing to do would be… But what if he wasn't – new theory, new possibility, check all of the options before committing to any one.

Need more data. Need to spend more time with John. Bring him along on more cases.

If he loved John Watson, then he would want this to be a date. Did he? Check, double-check, triple-check, until one day he realizes that it's too prevalent, too constant, to be an if any more.

Fact: Sherlock Holmes loves Dr. John Watson.

And all of a sudden, a new if makes its way into the picture:
What if John loved him back?

Need more data.

Check again and again. What if that look in his eyes means, "I love you"? What if he worries about Sherlock because he loves him too? What if he broke up with Sarah because…

Humans are complicated – solid deductions need more proof. Check again.

There is only so much checking he can do, before he gets the proof he was looking for. It wasn't in any of his possible worlds. For once, he didn't see it coming. But when John kissed him, Sherlock knew he finally had an answer.

Fact: Dr. John Watson loves Sherlock Holmes back.