In the base MAS mod, affection is a crucial hidden stat. You must spend real time with Monika, give her gifts, and compliment her to raise her affection level. Certain dialogues (like her confession about "deleting" other characters or her existential rants) only trigger at specific affection thresholds. outfits, room decorations, and even holiday events are time-gated.
A new button often appears in the UI providing quick access to submod-specific settings. monika after story extra everything submod work