You might have a few principles in mind, and that's great! The most direct answer is checks and balances. Checks and balances make sure that one branch does not have more power than another. The veto is an example of an executive branch check on the legislative branch.
Of course, since different branches are involved in a veto override, you might think "separation of powers." That's true, only Congress can pass bills. The president and executive branch must enforce the laws. That's why the president is given a role in the lawmaking process.
Now if you also are thinking "limited government," you deserve an award! Remember that the point of both separation of powers and checks and balances is to limit government power. This is done to protect citizens from tyranny.
The cognitive stage that Cassandra is illustrating is the
Formal Operations stage, this focuses on having to use reasoning and logical
thinking in which the person made use of hypothetical or even theoretical in
which Cassandra uses as she thought of hypothetical arguments.