Hopes this helps:
It all comes down to separation and distribution of powers. If one person has all the power, then it is easy for that person to abuse that power. For example, if Congress had the power to pass laws and determine if these laws were constitutional, then by definition, all laws passed by Congress would be constitutional. Similarly, each Senator is a check on the power of the other Senators, such that no one Senator can pass legislation. The House is a check on the Senate and vice versa. Presumably, the Constitution is a check on everybody in government.
