When it comes to jihadis, the "why" question is being asked again.
Why are young European Muslims drawn to the self-styled Islamic State (IS)? Why would a 14-year-old Dutch-born Muslim, whose family were given asylum after fleeing the Balkans, post a video online declaring his support for IS, adding he hoped to "cut Jews' heads off"?
A different "why" question: why are we still surprised? The presence in the West of young men and women drawn to the idea of terrorist jihadism was first noted after the attacks on the World Trade Centre. "Why?" was asked then. And again, after the 7/7 attacks in London. And now, as hundreds from across Europe join IS on its medieval rampage of head-cutting, rape and enslavement.
According to estimates by the Soufan group, a security consultancy staffed mostly by former FBI operatives, there are 12,000 IS fighters. They come from 81 countries, a quarter from the West. (A "how" question: how do the security people know these things? And if they know who they are, can't they stop them from going to Syria in the first place?)
Big social and psychological theories have been used to explain the phenomenon: immigrant alienation, the malign influence of the web, gender (young men love to fight and kill, don't they?).
A more specific reason, and one that might accurately describe some of the Western jihadists with IS, is that the rapidly shifting alliances of the war in Syria left some people stranded inside the jihad killing machine.
In an open letter published over the weekend on the Daily Beast website, Maajid Nawaz of the Quilliam Foundation addressed those IS fighters born in Europe who "went to Syria to fight that brutal dictator Assad, and now may be feeling like you joined the wrong war". IS, Mr Nawaz writes, seemed to them to be the most effective of the many groups fighting Bashar al-Assad - until they started killing their fellow Sunni Muslims. Mr Nawaz calls on the authorities to give these now reluctant jihadis a way out.
But whether we come up with a grand theory or specific explanations, it may be that the real reason why a few - very few - European Muslims become radical jihadis is outside our mindset. It relates to belief and faith. The wars of religion are over for us - thank, God. They are not over for a significant minority in the Muslim world.
A decade ago, I made a radio programme on radical British Muslims. I spent time with the radical preacher Omar Bakri Muhammad and interviewed some of his students. Much of what they said - like re-establishing the caliphate - struck me as nonsense.
The young men I interviewed were, for the most part, married with decent jobs. They were completely au fait with modernity: from technology to the political and social mechanics of modern society. But for them, returning to a pure Islam, eradicating apostasy and having their own state - all based on the way Bakri Muhammad taught them the Koran - trumped everything else.
I could not wrap my head around the dichotomy between their modernity and their medieval dream. But then, I do not have that kind of religious feeling.
There is a war raging for the soul of Islam in the Middle East. We in the West are primarily bystanders. The key to understanding "why" is to try to imagine a mind in which modern life and a medieval intensity and understanding of one's faith can co-exist.