It is always hard to answer "why", but here is how to ask "why."

  Why am      I        happy? 
  Why are    you     happy? 
  Why is      s/he   happy? 
  Why are     we     happy? 
  Why are     you    happy? 
  Why are    they   happy? 

Here are some others you will hear with "are":

How are you?     I am fine.
How old are you?    Women are not old.
Where are you from?   I am from here.

(Some English teachers do not like sentences to end with "from". But you will hear it most often that way...:)

                                                  End Lesson A Grammar

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