Dio might have been calmer around Raven for all we know, he's been missing for some time so I can understand why Dio is so gungho with Rood. It's like a person who haven't seen their best friend in what feels like a life time and they finally reunite, but with a twist that the friend doesn't remember anything and might end up in a deadly trap so Dios trying to stop Rood from being harmed.