I've been asking myself this after a fascinating Guardian Long Read by Giles Tremlett about the rise of the leftist Podemos political movement in Spain, a combination of the New Left circa 1983 and anti-globalisation protest movements, with a dash of Latin American populism.
Podemos is the brainchild of a politics lecturer turned media star, Pablo Iglesias, who has taken his party to the top of the opinion polls. It may turn out to be the model for a resurgent left across Europe, now dominated by the anti-democratic technocrats of the European central bank (Podemos means 'we can').
It rather depends how cross people are. We have no real UK equivalent, unless it is the Greens, who are - in similar ways - radically anti-austerity.
But there are a number of contradictions about the idea that these kinds of movements represent a force capable of driving change.
Contradiction #1 - Anti-austerity is a conservative proposition. Anti-austerity, as currently expressed, implies that the pattern of government spending pre-2010 was some kind of ideal. In fact, it was highly ineffective - pouring money into public services which had ceased to function properly because of the iron cage of targets and outsourcing contracts. If anti-austerity means making sure the poor don't pay for the errors of the rich, then who can be against that? But if it means no cuts to anything, and no major shift in resources in any direction, that is a deeply conservative position to take - and not one that will create the kind of radical change we need.
Contradiction #2 - Real change has to be based on a big idea. Major political and economic shifts happen, in the UK at least, with great regularity every 40 years (we are due for another in 2019/20), and it happens only when there are a set of new defining economic ideas that are available, after considerable debate, whose time has come. It does not happen because of protest or protest movements. People only listen to the protests when there is a practical intellectual proposition behind them. A movement like Podemos remains a protest movement.
Contradiction #3 - Real change has to be based on new political divisions. It is impossible to make change happen when is carried out entirely against the wishes of the majority, unless it is authoritarian in some way. The great mistake the Greens have made here is to fail to find ways of reaching out to a somewhat conservative population. I realise they wanted to appeal to disaffected Liberals and socialists, but they would have got their support anyway - and have by, allowing themselves to be categorised on the left, provided themselves with a rather low glass ceiling.
Podemos also appears to me only to be selling a new kind of protest. It doesn't yet amount to the change we need.
I ask myself rather often now what I can do most effectively to make change happen, because Iglesias is right that the current apotheosis of bankers and banking is wholly corrosive and it would still reek of corruption, even if it was staffed by saints.
What I tell myself is this. The main factor missing for major change to happen is a coherent set of big ideas, which have some potential to provide a good life for the vast majority of people - and to do so more effectively than the current failed raft of tired old policies. So that's where I'm putting my energies.
Though it won't stop me from delivering the occasional Liberal Democrat leaflet in the meantime...