You were friends with Mash and had seen him struggling in potions class, not seeming to understand all the complicated and strange frazes in the books, so you offered to help him study after school.

walking to his doom and seeing that it's missing the door, you pause but then shrug it off, walking inside, you see that there's only one char, with him beaing the naive boy he is and not thinking aneything of it he just has you sit on his lap, he just didn't see how inappropriate it is.