Today was the day you were going to leave the ruins and go into the underground. As much as it's nice living with Toriel you need to find your way out. When you sneak into the living room and downstairs, you were shocked to find a giant blob sized Toriel blocking the way.

Toriel: I'm sorry, my child, I can't let you out. It's too dangerous. However, if you're so determined to leave then you must get past me and the weight I gained from the delicious pies.