This article’s topic is なんで(nande) ,どうして (doushite), and なぜ (naze) which are translated as “Why?” or “How?”. It is one of the common words you hear in the daily conversation. Both words have the same meaning but have different connotations. We can't draw a clear line between their use, but here's a summary of the broad differences.
どうして is an emotional way of asking why
どうして is a way of saying “why” with the speaker's emotions and expresses a range of emotions such as frustration, surprise, regret and wonder.
Examples of どうして
Doushite Anna koto wo itte shimattanndarou.
Why did I say such things?
Omae wa doushite chikoku shitanda
Why are you late?
Kyujitsu wa doushite koumo mizikainda
Why are holidays so short?
In (1), it shows regret for saying such and such; in (2), one is questioning the other for being late; and in (3), it shows frustration and surprise.
どうして also can be used in the exclamation statement just like the English word “How” and “Why”. Example sentences (1) and (3) are typical examples of this, where the emotional side of the sentence is more evident than the reason.
なんで is also an emotional way of asking why
なんで is most commonly used in daily conversation compared to どうして. Both なんで and どうして involves speakers emotion but なんで is more colloquial (spoken language). なんで is often used by children and young people in everyday conversation. When you say "どうして" in spoken language, it sounds more formal or slightly more senior than "なんで". Also, "どうして" is a word with a sense of formality, so some people may feel uncomfortable using it in daily conversation.
Examples of なんで
Co-la wo nomuto nande shakkuri ga derundarou
Why do I get hiccups when I drink Coke?
Nande syukudai wo yaranaito ikenaino
Why do I have to do my homework?
Neko ha nande konnna ni kawaiinda
Why are cats so cute?
なぜ is a rational way of asking why
There is another word that means “why/how” which is なぜ (Naze) other than なんで and どうして. “なぜ" is a rational way of saying "why" that does not include the speaker's emotions, and is literal (written language). It gives the impression of being somewhat polite.
Examples of なぜ
Hito wa naze uranai wo shinjirunoka
Why do people believe in fortune-telling?
Shokuji no ato wa naze nemuku naru no darouka
Why do we get sleepy after a meal?
Ohuro ni hairu to naze mega samete shimaunoka
Why do people feel awake after taking a bath?
Naze naite irundesuka?
Why are you crying?
Since "なぜ" has almost no emotional overtones, "なぜ" is always used in a situation where one is asking for purely reason/cause, such as in the above examples (7) and (8) and (9), which are knowledge-seeking and academic situations. On the other hand, in example (10), you just want to know why you are crying, so if you use it for a person who is actually crying, you may come across as a cold, emotionless person. Sometimes it is better to use "なんで" and "どうして" in situations where emotion is required.
どうして = Emotional and colloquial
なんで = Emotional and colloquial, young people often use it, hard to use it for elderly
なぜ = Rational and literal, asking for just a reason
The differences between the three can be summarised as above, but of course, there are always exceptions. There are sentences that have “なぜ” and include emotion and there are cases where "なんで" can be used for the elderly. Please note that this is only a rough difference and should only be used as a reference.