Why does the mesh current equal for Thevenin's Equivalent
In short:
To find \$V_{TH}\$ voltage you need to disconnect the &quot;load&quot; resistance.



simulate this circuit &ndash; Schematic created using CircuitLab
And fins the \$V_{TH}\$ voltage.
But to be able to find \$R_{TH}\$. The Thevenin's equivalent resistance we need to short's all the voltage sources and open all the circuit current sources.
So, for your circuit it will look like this:



simulate this circuit