Assumer

When an individual tends to take on a certain form,attribute or aspect of things very quickly and abruptly;when he/she might think one thing just because you might have said another thing about that subject

Gosh billy was such an assumer yesterday when I texted him,he wanted to go to the movies but I told him I wanted to go with Sandra instead because it would've been fun,then he thought I was saying he wasn't fun?! Like c'mon billy.