Why do I feel this way

So I just finished the whole series of the tv the office and the last two episodes made me very emotional. I just couldn’t stop crying. For one the show was almost done. Then when they did all the past videos of Pam and Jim’s relationship and everyone’s great/ horrible moments. It’s been 2 days and I’m in this very depressing mood where I reminisce on everything in my life and how new things came come but they also will go. I don’t know why I feel this way. But I want to be happy