Did you ever notice how Archie seemed to treat Lionel like an actual son? In fact it seems like he treated him better than his own family. He was never fond of George and Louise which was well explained, but he really seemed to have quite a bit of respect for their son.

I think a big reason for that was Lionel used to placate Archie and "play along" with him, much to Meathead's chagrin. I think actor Mike Evans as Lionel was brilliant playing off of Carroll O'Connor. Makes me sad that Mike Evans passed away so long ago.

I think he's known Lionel for some time, probably longer than Mike's been around. Lionel did odd jobs around the neighborhood, so he's actually been helpful to Archie.

He made a fool out of Archie to the point that he really liked Lionel. Lionel was always pleasant and respectful toward Archie so that's why liked him.