Why are people great at first then become unbearable overtime?

I understand that hormones make you feel happy and in love initially when you first meet someone and start spending time together. However, in my experience most people end up being way different than they were when you first meet them- usually in a bad way.

Why can't we just be ourselves from the start? Why do people introduce me to their representative then when I meet the "real" them it's such a huge disappointment?
Why are people great at first then become unbearable overtime?
Why are people great at first then become unbearable overtime?
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