为什么 (wèi shén me) is the standard, neutral way to ask 'why?' in everyday Chinese. It's more common and less formal than 为何 (wèi hé), which is mostly used in writing or formal speech. Unlike 怎么 (zěn me), which can mean 'why' in some contexts but usually asks 'how' (manner/way), 为什么 always asks for a reason.