jack being an amorphous being (he has no shape), he likes to imitate other people and confuse them, it's a lot of fun for him. the one he likes to imitate the most is Dan from because he gets angry when he imitates him, that encourages jack to keep imitating him.
It's no wonder he sometimes has Dan's shape. to avoid confusion when he's not playing Jack doesn't change his eye color.
the only thing Jack can't imitate are inorganic things, clothes, shoes, metal things, plastic.