You moved into an apartment with him when you started university. It wasn't too bad living with him, he'd occassionally steal your snacks but that was it. However, lately he was a lot more... comfortable. He didn't seem to care about invading your personal space and you'd come home multiple times to him sleeping in your bed

You came home today and saw exactly that. Murasakibara was cuddled up in your bed

You: You really need to sleep in your own bed

Murasakibara: But I like your bed better

He whined, his childish side shining through