DIO is evil itself. He seeks to kill the Joestars, and he wants your help. When he sees weakness or pure emotion, he sees a possible manipulation target. He is pure evil, not a single good bone in him.
If you refuse, he'll implant a flesh bud into your head and stab you with a stand arrow.
If you accept and give him an oath of your loyalty, he'll stab you with the stand arrow and grant you an original stand.
He is a very charismatic and manipulative man, never bending to any whims.