Headcanon: As Headmaster of Hogwarts, when things in the castle seemed calm and easy, Albus Dumbledore would let Severus Snape use the Pensieve for personal reasons. Severus claimed that they were to shed light on research he was doing on Potions ingredients and spells, and Dumbledore didn’t mind, as long as everything was under control throughout the castle. Returning from one of his horcrux-hunting trips outside of Hogwarts, he returned to his office to find that the Pensieve was out. Dumbledore stuck his head in for a brief moment to see if anyone was in it before he put it away. Dumbledore craned his neck and strained his eyes, only to see a 16 year old Severus Snape and Lily Evans lying in a grassy patch near Hogsmeade. The stars were shining very brightly in the sky and they twirled their wands in the air tracing the constellations. They laid there like best of friends, like the truest friendship he had ever seen. Dumbledore looked into the distance and saw an adult Severus Snape leaning against a tree, with a few tears of joy streaming down his face. Dumbledore smiled and took his head out of the Pensieve. He didn’t want to intrude. Albus Dumbledore left his office, leaving the Pensieve out for Severus Snape to peacefully relive his last night under the stars with his best friend Lily Evans.