Leaving aside the specific motivations of protesters (of whom there seem to have been very few) and criminals (of whom there seem to have been many), I've picked out what seem to me to be some of the main causes:
*An exaggerated sense of hopelessness, resulting from left- and right-wing political attitudes towards the young and the poor. Relentlessly portrayed as either victims of society, or parasites on it, many young people grow up seeing themselves as destined for nothing better than benefit dependence and/or criminality. It's important to note here that the problem - as far as these riots are concerned - is not material poverty or social immobility. It is, rather, that there is a profoundly negative image imposed by society at large, which, for some young people, is more easily dealt with by rejecting society and its norms altogether.
*The relentless influence of 'gangsta' ideals in urban culture, promoting the glorification of criminality and the obsession with material wealth. It's worth noting that this influence is far from uniform - there are plenty of UK hip-hop artists (for example) who argue against such malign ideals. It is, perhaps, more of a reflection on our media culture that those more positive voices tend to be ignored in favour of their more glamorous, bling-draped contemporaries. In any case, I believe that the 'gangsta' influence is instrumental in nurturing the attitudes behind much of the semi-organised looting we've seen over recent days.
*The repeated and massive failure of the Metropolitan Police. Policing in rougher areas of London is regarded by many as having only two settings: hyper-aggressive and non-existent. Corruption is thought to be widespread, and to begin at the very top (the latter being proven recently through the hacking scandal). A leadership culture has developed wherein accepting any responsibility is career suicide, and the principal goal is always to deny and cover up any failings on the part of particular officers or the Met in general. With regards to the weekend's events: on the one hand, we could see long-festering enmity towards the Met begin to overflow. On the other hand, we once again saw the service make a mess of things and then fail to take any responsibility for its mistakes. Indeed, watching TV and listening to the radio these last days, I've heard a relentless flow of self-justification from the Met and its supporters, coupled with repeated attempts to shift any blame onto the policies of this government or the previous one. After the dust settles on what is, after all, just the latest in a very long line of disasters over which Scotland Yard has presided, I would suggest that serious questions need to be asked about whether the Met, in its present form, can any longer be regarded as fit for purpose.
(With regards to other places where rioting has broken out, I am unsure whether their police forces should also be looked at with a critical eye. Clearly, the problems spread across and then out of London as gangs recognised the opportunity. It may be that the rest simply had to deal with the knock-on effects of the Met's failure. On the other hand, it could be that legitimate local grievances with the police were also a factor in encouraging the riots.)