Since we can say "Why can we grow taller?", "Why cannot we grow taller?" is a logical and properly written negative. We don't say "Why we can grow taller?" so the construct should not be "Why we cannot grow taller?" The reason is that auxiliaries should come before the subject to make an interrogative. "why" can be compared to an old Latin form qui, an ablative form, meaning how.

Understanding the Context

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 ...". 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 ...

Key Insights

11 Why is it that everybody wants to help me whenever I need someone's help? Why does everybody want to help me whenever I need someone's help? Can you please explain to me the difference in meaning between these two questions? I don't see it. Why is a just a rather odd wh -word.

Final Thoughts

Its distribution is very limited -- it can only have the word reason as its antecedent, and since it's never the subject it's always deletable. Consequently it behaves strangely, as you and others point out. Can "why" be a conjunction? - English Language & Usage Stack Exchange For why' can be idiomatic in certain contexts, but it sounds rather old-fashioned. Googling 'for why' (in quotes) I discovered that there was a single word 'forwhy' in Middle English. grammaticality - Is it incorrect to say, "Why cannot....?" - English ...