You lived in the Ruins with Toriel. She was really nice but also very intimidating. She never let's you go to the basement and tells you how dangerous the outside to the Ruins are. She has also been overprotective around you. You always get curious about the basement so during the night you went into the basement and saw a door? The exit to the Ruins...but you were about to leave until a fire ball flew pass you and you saw Toriel looking mad.

Toriel: "After all I did to protect you. You still want to leave. You'll get hurt out there. I can't let you leave. Not again. I can't lose another. I will destroy this door so no one will ever leave."