Coworker is extremely upset that I'm quitting my job?

At first, when I told her that I was leaving for a different opportunity, she told me she would miss me but that she was happy for me. It really seemed genuine. However, during my last week at work, she has completely changed and is now snippy with me, won't let me finish sentences and it's really uncomfortable. We got along before but it's time for me to move to a better opportunity and this seems to upset her. Why do coworkers act like this when you leave a job?
Coworker is extremely upset that I'm quitting my job?
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