I've been giving it some thought myself (even before the attacks this morning), I just want to note this is theorising on my part, I'm not claiming this as fact, and that it's not my intention to cause offence;
I can imagine there are quite a few places in the world where communities are living almost side-by-side but with a gap (sometimes huge) in terms of well being and general contentment, be it health, wealth e.t.c. It stands to reason that people who are less well-off may well feel resentment to those on the other side. In most cases, this wouldn't result in anything as serious as what we've seen today and in the past, but when you take radical Islam into account (and their promises of an eternity in paradise for blowing yourself up along with the "infidels") it tilts the scales considerably.
Let's not forget, when you're struggling to make ends meet and the future in general looks utterly bleak, suicidal tendencies may already start to creep in. Whilst the reasoning from going to taking you own life to those of everyday people in the process is a massive gulf for me, you and just about every right-minded person out there, when you're indoctrinated into a way of thinking such as those that Daesh teach, it's merely one small step I fear.
Revenge is another to consider; your family getting wiped out by a drone/air strike (a morbid example) is a powerful trigger to "hit back"; for some they won't even attempt to make the distinction between whether it was the US, Russia... just to hit back at "them", I can imagine the ring-leaders of the extremists are very good at blurring the lines in that regard.
I also remember reading an article or two a little while ago about the problems second-generation Muslims have with growing up in "the west", leading to something of a conflict of identity, I'll try to find it later. Overall though, I fear it's very much a case of emotion/passion clouding reason; in most cases I think they're too far gone to actually stop and consider what it is they're doing, it's all bottled up until the very end.
As an aside, some of the reports I saw of Salah Abdeslam's arrest mentioned that he was "co-operating with police" (specifically, the police had released that info). In hindsight, that now seems very ill-judged indeed, as it may well have spurned today's cell into action in case Salah had anything on them.
