WHY versus BECAUSE

WHY is used in questions.

• Why do you like that stupid guy?
• Why didn't you me to the party?

See more examples here.

WHY  is also used to affirmative sentences to mean "this is the reason why"

BECAUSE is used to answer, never to ask.

Question:    Why are you late?  

Answer:       I am late because I went to bed late.

Question:    Why is the window open?

Answer:        The window is open because it's very hot here.