Alexander Hamilton was very concerned about what might occur if the Constitution wasn’t adopted. He had seen firsthand the difficulties this country faced with the weak plan of government created by the Articles of Confederation. He worried that our financial issues would make it difficult for us to succeed as a country. He worried about our government being able to keep order at home. He feared we would continue to be viewed as a weak country in the eyes of other countries. He also was concerned our trade would be negatively impacted.
Alexander Hamilton believed that if the federal government wasn’t a strong government, as created by the Constitution, the states would act like independent units. They would do what was best for them and not what best for the country. This would only weaken our country and endanger our future. It would be very difficult to get much done. Alexander Hamilton believed it was essential to ratify the Constitution.
Basically, Hamilton feared that the country would be broken up into a lot of semi-independent states that would be run in such a way that the country's economy would suffer badly.
Hamilton wanted a big country with states that were not very independent. He wanted trade to flow freely between the states. He wanted it to be hard for the common people to take control of the government, preferring that elites have more control.
Hamilton feared that without the Constitution all the states would be practically independent and would not trade with one another easily. He also worried that the common people would take over the state governments and enact bad (in him mind) laws like the ones being demanded by the rebels in Shays's Rebellion. Between these two factors, the economy of the country would be ruined. The US would have ended up as a loose confederation in which both the states and the country as a whole had very weak economies.