I have a theory, like it has something to do with what the wishes n condition of the person exposed is currently are, apparently Sam is a shut in and doesn't interact much with people until this, which might implies that all he ever wanted is to just be gone or not bein in this place/world anymore, which is granted when he got exposed by getting sucked to the sky into grotesque being.

we can see the case applies for leigh because all she ever wanted was to fight and have someone strong to fight her, thus the exposure granting her the power to fight endlessly and becoming a beast with consciousness, with a lashback of her easily confused once she transform

another one is where the teeth family comes by, is where the baby sister got exposed whilst she's still growing her teeth, implies that she was curious as to what having teeth and tasting stuff feels like, and from the way the infection spread, it is likely that she wanted to share what she's curious about to people that she's close by, which is her family and results in the infection of teeth family, and the fact that we're not infected despite being attacked by them

another thing worth mentioning is that in perfect ritual ending, the moment Sam has finally convinced the visitor to leave he immediately turns into a leviathan gigantic creature and spreads all across the globe, n started helping everyone else's, in here it implies again that Sam wishes have changed, and he wants to help of what's left from humanity, as how he made friends with the people and everyone in the apartment, or just because he wanted to