Hamm, who has dated the actress for 14 years, told The AP that he admires how brutally honest Westfeldt is at all times.
"She's an open book," the Mad Men star explained. "'There's not a lot of subterfuge or deflection in her, probably to her detriment. Sometimes the press can be a nasty beast, no offence."
He added of Westfeldt's honesty: "She is the most open-hearted, sweetest most loving person that I know. And I think that's pretty obvious in the work that she does."
The Mad Men actor recently revealed that he and Westfeldt aren't planning to have kids anytime soon because they are both currently focused on their careers.
Hamm and Westfeldt co-star with Kristen Wiig, Adam Scott and Maya Rudolph in Friends with Kids, which is now playing in US cinemas and opens on June 29 in the UK.
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Watch the Friends with Kids trailer below: