The actor told TV Line that the unwavering support of wife Trudy (Alison Brie) simply isn't enough to please the eternally unhappy Pete.
"Trudy's been really great for him, and he continues to kind of not be satisfied with that," Kartheiser noted. "But I think anyone who's been married or in a relationship for more than five years realises that that's when the real work begins.
"He's still relatively a young man, although he's passing into middle age now, and he wants more than just 'steady and secure'; he wants to feel alive and important and wanted. And you can't really get that from someone you've had sex with a thousand times."
Kartheiser went on to discuss the growing antagonism between Pete and his older and wealthier business partner Roger Sterling (John Slattery).
"I think the student is now the competitor," he declared. "I still think there are things he could probably learn from Roger, but business has changed since Roger was out getting accounts. And maybe if Roger wasn't so damned pretentious, [they'd get on]."
Kartheiser recently insisted that he still has hope that Pete will end up happy before the end of the series.
Mad Men airs on Sundays on AMC in the US, and Tuesdays on Sky Atlantic in the UK.
> Mad Men Teyonah Parris: 'I had no idea about Dawn's back-story'
> Mad Men star Vincent Kartheiser: 'Pete, Peggy have healed wounds'
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