convenience. she flirted with him for 5 years, always saying she could be a quiet side dish.
One day, arnold wasn't getting laid and he had a few scotches in him. She took it... bent over in front and arnold did what he did.
he probably discovered it was easy, discreet, convenient, and available anytime he wanted. She wasn't a looker, but someone to suck him off under the desk or take his load anytime he wanted... under the same roof without suspicion.
Maybe he wasn't allowed to hire hot maids?