Funny thing about taking economies as a whole is that spending and earning are connected. Consuming more today lets the person you bought those goods from produce more goods tomorrow. However that guy you bought from needs to put that additional money into the same economy it came from in order to grow it, if he puts it outside the country or something then sooner or later too much money gets siphoned out of the economy and those consumers become unable to consume and thus generate income for the producer. That means the producer needs to get rid of his surplus capacity which means firing workers, those workers are then unable to consume. Get enough people unable to consume and the economy grinds to a halt. Neither the govt nor the rich must be allowed to accumulate too much wealth without spending it into the local economy again or the system grinds to a halt and the economy which is measured by the flow of money, not the total of it that is available, crashes.
Giving poor people govt handouts is indirectly subsidizing the businesses of the area (where else do you think they're going to put that money?).
Two possible threats are when the govt decides it must save money instead of reinvesting the money it takes via taxes and such, then it acts as a brake on the economy like in Greece.
The other threat is that one point in the chain of spenders and earners is unable to spend, e.g. when rich people cannot find investment opportunities and thus decide to sit on their money. That also acts as a brake on the economy because the flow of money gets stuck.
Failures amplify themselves as Greece has demonstrated, they increased taxes while cutting spending, thus increasing the friction on the economy. This included salary cuts and firings. That means fewer consumers. That means less income for the businesses. The economy slowed and the GDP collapsed. And then Greece stood there with even lower tax incomes, even higher debt to GDP ratios and even more problems.