It was Leonard who realised that Sheldon and Penny were going to get married.
The two have been together for just over a year now, and they are both happy, he knows. They haven't had an argument since Penny accidentally smudged an important equation on Sheldon's whiteboard – and seeing as it was because he had pushed her up against it (or so Leonard had heard her yell across the hallway) it was hardly her fault at all. Since then, they hadn't fought at all, and this was why Leonard found Sheldon's current behaviour to be so strange. He had been avoiding Penny for the last few days and had barely left his room, except for work and bathroom trips. Their anniversary was this coming Friday and Leonard couldn't figure out why Sheldon was acting the way he was – so he decided to interrogate his best friend.
He knocked on Sheldon's door, only to receive a 'please do not disturb me Leonard' and then silence. Leonard ignored Sheldon's request and opened the door. He was sitting on his bed and at Leonard's entrance gave a high pitched yelp, shoving something under his pillow.
But Leonard had seen what he had been holding.
He had seen the almost scared reverence on Sheldon's face as he looked at the blue velvet box in his hand.
He also saw the apprehension that was shining in his friend's uneasy blue eyes as he looked at him guiltily.
Leonard took a deep breathe and slowly let it go.
Let Penny go, once and for all.
The silence was broken by Sheldon standing up, stuttering.
"Leonard I… I can expl-"
"Explain? There's nothing to explain."
Sheldon's face fell, and Leonard took a step forward.
"You were just about to ask me to be your best man, weren't you?" he asked, cocking an eyebrow playfully.
Sheldon's shoulders dropped from their previous tense position as a cautious smile crept onto his face.
"Really Leonard – I didn't think I needed to ask. You are the logical choice after all."
The two friends eyes met and they smiled in shared understanding.
Sheldon was getting married.
And Leonard couldn't be happier for him.