Sherlock would have never thought he would act on jealously. Well, for that matter he would have never thought he would get jealous.
He was a genius. He had no reason to wish to be like any of the idiots that surrounded him. He felt no desire to be in any of their position. He was content with own his mind and his work.
That is.
Until he met a certain doctor.
John Watson had hit him upon the head with a whirlwind of emotions that sent him falling.
Never before in his life had Sherlock felt the need to impress someone, or to cause someone to smile or giggle.
It literally almost made him faint when he felt the urge to kiss him for the first time.

He would ask himself all the time. What is happening to me?

But it always came to the same conclusion. Love. Me? Falling in love. Nonsense.
Although, he knew that's exactly what it was.
He felt himself tumbling into a bottomless pit called Love and it was all John Watson's fault.
He wasn't sure if John loved him back. He tried to read him, deduce him, and figure it out. But John was always the one person to mix him up even when he thought he had finally figured it out.
Countless times Sherlock thought John would go running. Like the first time John saw ears in the fridge. Or when Sherlock shot holes in the wall. Or the fact that Sherlock never bought any milk and yelled at him constantly because he was so bored.
Other people go running.
But not him.
Instead John smiled at him, and laughed, and joked.
And stayed.
Who on Earth ever wished to stay with Sherlock Holmes?
But the fact that John did, meant all the world to Sherlock.
This man, with a bad taste in jumpers, a goofy grin, and a love for adventure, had changed Sherlock's life.
So of course he felt jealous when he thought he might lose it to some man standing across from him.
John had made Sherlock take a walk with him to get some fresh air, and left Sherlock on a park bench to get some nearby coffee. After about 15 minutes Sherlock was getting curious and agitated, let alone bored.
So he set out to find where his Doctor ran off to.
And as it turns out he ran into a another man.
5"7. Late 30s Curly brown hair, green eyes, cinnamon skin. Divorced (but not recent), right handed, nearsighted, and owns a Siamese cat.

Dull.

Part-time Barista boy, but is well off. He inherited a lot of money. Then what was he doing serving coffee?

Picking up other people's flatmates is what he was doing.
This man was nothing exciting, but apparently he was charming, because whatever he was saying had John ogling all over him.

Or to Sherlock it seemed like ogling
Slamming his way through the glass doors Sherlock marched right up to John and gripped his arm.
Alarmed, the smaller man looked up at the crazy consulting detective trying to figure out when he'd come in.
"Sherlock? What are you doing?"
Sherlock held a cold stare with the barista.
Rolling his eyes he looked towards his companion.
"We have a case."
"Already?"
"Yes, now let's go."
"Alright okay."
Visibly annoyed, John picked up his coffee. Glancing back the barista he smiled, "Thanks again."
"No problem at all."
Seeing a smug smirk on the barista's face Sherlock boomed, "No wonder your husband left you, after you picked up that STD from your cheap prostitute."

Shocked and embarrassed John dragged Sherlock out of the little shop muttering apologies.

When they got outside, John immediately sent the taller man an icy stare before yelling, "What the hell is wrong with you?"

Sherlock was walking briskly, so much so, that John had to almost jog to keep up with him.

Pointing back to where they came from he asked, "What exactly was that. I was coming. I was just getting coffee."

With annoyance Sherlock huffed, "You should be thanking me, John."

"Thanking you for what? Making me look bad by hanging with a complete arse hole."

"Honestly, you can't be this upset. He wasn't worth your time anyway. And he had a disease."

"What. In the hell. Are you talking about. Worth my time? We were talking."

"You were being flirtatious."

"I wasn't flirting. Just easy conversation, and hey! What does that have anything to do with you if I flirt with him or not?"

That drew the line. Halting Sherlock grabbed John by the arm and growled, "It's got everything to do with me."

Standing dumbfounded he looked up into the taller man's eyes shaking his head slightly.

Letting him go slowly Sherlock turned around leaving a heavy silence between them.

Waving for a cab they got in slowly.

"221B Baker street," Sherlock ordered.

"I thought you said we had a case."

"I just made up my mind that it wasn't worth my time."

"There wasn't a case was there?"

He remained silent.

John stared at him with anger and confusion, "What did you mean he's got everything to do with you? Sherlock you're sounding jealous."

Sherlock scoffed and turned to hide his blush.

John squealed, "Really? You're jealous of him? For what? Because I left you for a few minutes to talk to him? You act as if I wasn't coming back."

Sherlock muttered, "This isn't amusing."

"Of course it is! Why would the world's smartest man feel the need to have these feelings over a lousy coffee maker?"

"It's not like that."

"Explain it to me then."

Getting out of the cab, Sherlock unlocked the doors and flew up the stairs.

Swiftly paying the cabbie and trying to keep up John yelled, "No, your not going to ignore me. You wanna act like an arse hole then you better explain yourself!"

Hearing the other man's stomps up the stairs Sherlock flung himself across the couch face down.

"Sherlock!"

"John."

Growling John sat across from him waiting in frustrated patience.

Sensing his presence Sherlock lifted his face and scrunched his eyebrows.

John relaxed himself a little,"Ready to talk now."

"There's nothing to say."

John slammed his face into his hands.

"Why do you always do this? You take me away from other people and then act like it's normal. Why, Sherlock? Do you have some sort of issue with me talking to other people? Why won't you ever let me socialize? Why is it that you always try and break me from other people before we even bond?"

Jumping up from off the couch Sherlock screamed, "Because I don't want you to go anywhere else!"

Breathing hard and red-faced John was stunned silent.

Lowering his voice to barely above a whisper Sherlock continued, "If you meet someone else, then could get a sense of something better, and then you will be gone. And…"

Scratching his head and walking towards the window he spoke softly, "And I just can't let that happen."

Getting up from where he was sitting John slowly met Sherlock at the window. Placing his head tentatively on the taller man's shoulder, John let out a deep breath.

In the reflection of the glass Sherlock could see some of John's body cradling his own, otherwise he wouldn't have actually believed it was happening.

John whispered, "I know that this isn't just about making other friends...You love me don't you?"

Closing his eyes to fight the tears Sherlock slowly turned around to be face to face with him, "I'm afraid John…"

Raising his hand John wiped the lonely tear from Sherlock eye and cradled his cheek, "I will never leave you."

"Are you saying you feel just like I do?"

He smirked, "You mean afraid? A little, I don't want to ruin our friendship. But I don't ever see myself with anyone else...Sherlock, as much as you drive me crazy...is as much as I love you. And I mean that."

Sherlock grinned like a maniac adding, "You're such a sap."

Giggling John stepped back, "I'm a sap? Really?"

Joining him his giggles he lightened up a bit and with a much needed and longed for embrace Sherlock whispered in his ear, "I love you too. Promise you will never leave."

"As long as you promise you will always stay."

"I promise."

Releasing the smaller man while keeping one arm wrapped around his waist, Sherlock ran a finger over the face of the only man he will ever love, and he held his hand there.

"John,"

"Hm?"

"May I kiss yo-"

And before he could finish John grabbed his shirt met him into a passionate kiss.