Because Reed was nice to nearly everyone. He was a nice guy. Neptune had made some alterations to that while in charge of their shared vessel, because he wasn't always nice. That made things difficult. Reed couldn't explain it away so easily. So he didn't know how he was going to explain himself and the god to Oliver.
Reed sighed and scratched the back of his neck. "I told her it was fine if she wanted to tell her friends. People she trusts." So in that sense Reed should have been expecting this. He was still socked that Lydia believed him, even after Neptune's rather convincing demonstration. But her friends wouldn't necessarily buy it. "Uh.. do you want to.. I don't know.. talk for a bit?"
Gesturing up the road, Reed headed in that direction. Lydia could wait a few minutes at least, especially when she wasn't even expecting him. He parked himself on a bench and waited for Oliver to join him. "I don't expect you to believe me," he started. "I hardly believed it myself at first. I thought maybe I was suffering from some sort of psychotic break. But I don't have multiple personalities. D.I.D. I think they call it. Dissociative Identity Disorder. That's not me. Neptune is a separate soul who just happened to wake up the other week, and now we're sharing my body." Which was shitty for both of them.