People Change??

It always amazes me when someone or something changes so drastically. I don't understand how a person can be one thing and practically overnight they are someone else. Does that mean they were fake before? Or are they fake now? I understand that when you are young you are still trying to find out who you are as a person. But that being said, you change gradually over the years. It's not all of the sudden. I will never understand it. I give up.