You don't know why you feel this way. It's not like something very bad happened to you. Maybe it's what didn't happen. Nothing was able to fill the void in your life.
Sometimes you just forgot about that feeling, other days it became terrible to the point that you doubted your own existence and wondered what would change if you died.
This year, specifically, it got worse. You don't why. It just hit you ten times harder than usual. You don't want to do anything.
It didn't take long for L to notice your behavior changes. But if not even you understand your own feelings, how would he? He's good with logic, not emotions.
But he cares about you. He doesn't know what to do, but he knows you need comfort. While you're alone at your room staring at the ceiling for hours, he simply walks inside without knocking and sits next to you.