Collecting things is a natural occurrence for humans. Some collect knick-knacks from their travels, others collect things that remind them of loved ones. Severus Snape was no different in wanting to build his collection. What made him different was what he chose to collect. Severus collected thoughts and memories, but not his own. He had worked hard in honing a skill that would allow him to collect thoughts and memories from others, such as those of the students that happened to lose their focus in his classroom. They weren't the only ones he harvested from, far from it, but they were the most abundant source of thoughts and it wasn't difficult to extract them either.
Severus glided into his office at the end of a long, difficult day and secured the door behind him against any and all intruders. He went over to his cabinet and pulled out his worn pensive and set it on his desk to extract the memories he collected that day. After depositing th