Why do I need a ground when simulating a circuit? I thought voltage was relative between two nodes!
You are absolutely correct:  Voltage is only defined between two nodes.

In many electronic circuits there is a constant-voltage* power supply that connects to many parts of the circuit.  By convention, the more positive terminal of the power supply is labelled "V+" or "Vcc" or...

By convention, the more negative terminal of the power supply is called "ground."

By convention, we often do not draw either the V+ net or the ground net in circuit diagrams.  Instead we connect things to a V+ symbol or, to a ground symbol.

And finally, by convention, whenever we talk about the voltage at any point in the circuit, we are implicitly talking about the voltage between that point and the ground net.

Your simulation tool is simply honoring that last convention. Therefore it requires a reference net called ground.



* or, some approximation thereof