why

3. used to give or talk about a reason
• That's why I left so early.
• I know you did it— I just want to know why.
• The reason why the injection needs repeating every year is that the virus changes.
Word Origin:
Old English hwī, hwȳ ‘by what cause’, instrumental case of hwæt ‘what’, of Germanic origin.

Idioms: why ever ▪ why not? ▪ whys and wherefores

exclamation (old-fashioned or NAmE) used to express surprise, lack of patience, etc.
• Why Jane, it's you!
• Why, it's easy— a child could do it!
Word Origin:
Old English hwī, hwȳ ‘by what cause’, instrumental case of hwæt ‘what’, of Germanic origin.

noun
Word Origin:
Old English hwī, hwȳ ‘by what cause’, instrumental case of hwæt ‘what’, of Germanic origin.

Cambridge Advanced Learner's Dictionary

Cambridge Advanced Learner's Dictionary - 4th Edition

why / waɪ / noun

the whys and (the) wherefores the reasons for something:

I know very little about the whys and the wherefores of the situation.

Collins Advanced Learner’s English Dictionary

why

[(h)wa͟ɪ]
♦
(The conjunction and the pronoun are usually pronounced [(h)waɪ].)
1) QUEST You use why in questions when you ask about the reasons for something.
  Why hasn't he brought the whisky?...
  Why didn't he stop me?...
  Why can't I remember the exact year we married?...
  `I just want to see him.' - `Why?'...
  Why should I leave?
2) CONJ-SUBORD You use why at the beginning of a clause in which you talk about the reasons for something.
  He still could not throw any further light on why the elevator could have become jammed...
  Experts wonder why the US government is not taking similarly strong actions against AIDS in this country...
  I can't understand why they don't want us.
ADV: ADV after v, be ADV
Why is also an adverb. I don't know why... It's obvious why... Here's why.
3) PRON-REL You use why to introduce a relative clause after the word `reason'.
  There's a reason why women don't read this stuff; it's not funny...
  Unless you're ill, there's no reason why you can't get those 15 minutes of walking in daily.
ADV: n ADV
Why is also an adverb. He confirmed that the city had been closed to foreigners, but gave no reason why.
4) QUEST You use why with `not' in questions in order to introduce a suggestion.
  Why not give Claire a call?...
  Why don't you come home with me until you sort things out?...
  Why don't we talk it through?
5) QUEST (feelings) You use why with `not' in questions in order to express your annoyance or anger.
  Why don't you look where you're going?...
  Why don't they just leave it alone?
6) CONVENTION (formulae) You say why not in order to agree with what someone has suggested.
  `Want to spend the afternoon with me?' - `Why not?'...
  `Shall I tell them about poor Mrs Blair?' - `Why not?'
7) EXCLAM (feelings) People say `Why!' at the beginning of a sentence when they are surprised, shocked, or angry. [mainly AM]
  Why hello, Tom...
  Why, this is nothing but common vegetable soup...
  Why, I wouldn't give the end off one of my fingers for all the money you've got!
8) the whys and wherefores → see wherefores