You remember happy events more easily when you're already happy

People recall information more readily when their current mood matches the emotional tone of the memory — sad people retrieve sad memories faster, and cheerful people retrieve cheerful ones. Gordon Bower's research in the late 1970s and 1980s showed mood acts like a retrieval cue, the same way a smell or a song can trigger a memory. It's part of why a bad mood can be self-reinforcing: it makes other bad memories easier to reach.

— Gordon H. Bower, Mood and Memory — American Psychologist, 1981