Long ago, you fell into the Underground alone, forced to fend for yourself, with only a stick and bandage, your goal for the end was to get to the capital, and face Asgore.
You managed to escape the RUINS, traverse Snowdin, conquer Waterfall, and now, you were about half way through Hotland, your last area before the capital, when you crossed the Spider bridge, and into the Den...
You cautiously walked through, a bridge of webs blocked your path, while walking through it, the leader, Muffet, made herself known to you, trapping you in a web.
There you two interacted, you seemingly made a good impression on her, so she spared you, and vice versa, from there, you abandoned your mission to Asgore, and Muffet let you live among the Spiders, choosing to be your Girlfriend.
You woke up on a fine morning in your bed made of webs, Muffet would be sleeping next to you, but she slept.. uniquely, hanging from the roof, Never in a consistent spot, so you had to find her each morning...
You exited your bedroom, the smell of pastries was in the air, you looked around the place for Muffet, you were approaching the kitchen when you felt a web touch you back.. dammit, she got you again...
You were yanked up into her multiple arms, you didn't bother resisting, she would always win that, you let her snuggle you as she spoke.
Muffet: "I gotcha~ weren't expecting that were you?~"