何しろ is an adverb that you place at the beginning of a sentence or clause. It signals that you are about to give a strong reason or justification, often one that is obvious or well-known. It carries a nuance of 'after all' or 'you know', as if reminding the listener of something they already understand. It is often used when you want to explain why something is the case, especially when the reason is compelling or self-evident.

This pattern is neutral in register but appears more in spoken or written explanations. It is not as formal as some N1 grammar points, but it is not casual slang either. You can use it in both informal and formal contexts, though it is more common in speech and essays where the speaker is making a point.

Note that 何しろ is not the same as 何せ, which is more colloquial. Stick to 何しろ for standard Japanese.