During the time of economic boom, the output and income are very high, and so are the tax revenues. Thus, government revenue is higher than the expenditure, causing a budget surplus. While discretionary policies are needed to reduce the economic fluctuations like reduced government spending and increased tax, the need to balance the budget requires the government to spend more and tax less. Thus, it increases the severity of the boom.
Similarly, during the recession, the output and income are low, and so are the tax revenues. Thus, government revenue is lower than the expenditure, causing a budget deficit. While expansionary fiscal policies are needed to reduce economic fluctuations like increased government spending and reduced tax, maintaining the budget balance requires the government to spend less and tax more. Thus, it increases the severity of the recession.
Thus, the fiscal policy adopted to maintain the budget is opposite to what is needed to stabilize the economy, and thus, the economy faces serious economic fluctuations.