Why does Clark hate Lewis so much?

Just because he retired early? For an old person, the need to retire should be an understandable thing. But from the way Walter Matthau portrays the character, it is as if Lewis has shot his firstborn or run off with his wife, or something of that nature. I know the comedy works in such a way that all the gags and punchlines require the characters to behave in certain ways. But still, Clark's behavior goes beyond being just a grumpy old man to the point of appearing to be suffering from serious dementia that cries out for special treatment.