Johnlock Prompt! In their most recent case, the primary suspect's alibi is called into question by Sherlock. She said she was making out with someone when the victim was murdered, but her lipstick is clearly perfectly in place when the boys get there to question her. She says that her lipstick is long-wearing to the point of semi-permanent and though another person corroborates her story, Sherlock is doubtful.

Sherlock has to test this lipstick's durability for himself. Since the friction and textures involved in making out are nearly impossible to replicate altogether, he turns to his trusty assistant for this experiment. Maybe he's just using this as an excuse, maybe this awakens something in him, maybe John gets a bit too enthusiastic seeing Sherlock's mouth all highlighted like that, doesn't really matter to me.

TL:DR All I really care about is seeing Sherlock put on a longwear lipstick and make out with John for scientific purposes.