Answer the question about text:

And ice flows through Dean 's veins as every muscle in his body locks , freezing him in place . Sam steps forward , a shadow taking on stark definition as he steps into the circle of light and Dean feels his stomach twist and roll unpleasantly . His brother 's face is blank marble , expressionless and unreadable . His clothes look lived in , creased and marked and , as he steps closer , there 's a sickly sweet aroma wrapped around him , pervasive and cloying that makes Dean 's eyes water .

What may be the reason Dean 's stomach is twisting and rolling ?
He is seeing his sibling in a non - human form and it makes him sick .