Kim Kardashian Explains Why She Rarely Smiles

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Why so serious, Kim Kardashian?

The 34-year-old reality star is rarely seen showing off her pearly whites and Kim has a simple reason why she’s often never smiling or laughing: wrinkles. Yup, wrinkles.

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Kim made the revelation in a tweet Monday along with a candid black-and-white photo from her family’s Christmas party of her and friend Shamari Maurice laughing it up and enjoying each other’s company.

While Kim -- ET's No. 1 headline-making celebrity of the year -- was all smiles for the holidays, the same can’t be said for her adorable 1-year-old daughter North West when she met Santa for the first time.

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"North West is still warming up to Santa," Kim wrote in the caption on the Instagram pic showing North not too thrilled about Santa’s presence.

The past 12 months have been busy for Kim, from her wedding to Kanye West to landing on the cover of Vogue to "breaking the Internet" with her Paper Magazine cover. Find out why 2014 may have been her biggest year so far!