"why" can be compared to an old Latin form qui, an ablative form, meaning how. Today "why" is used as a question word to ask the reason or purpose of something. This use might be explained from a formula such as "How does it come that ...".

Understanding the Context

If you meet an old friend of yours, whom you never expected to meet in town, you can express your surprise by saying: Why, it's Jim! This why in the ... The grass is wet because it rained last night. This seems the simplest and most elegant expression of the meaning.

Key Insights

I am always suspicious of "reason (s)" and "why" being next to each other. There can be reasons for things but there is usually a better way of expressing "reasons why". To have a question, you need inversion: "why is there no roof?" Or "why isn't there a roof?" Edwin Ashworth: if there's no punctuation, I'd also read a headline such as "why X went to dinner with Y" as a relative clause.