Why is diode D2 affected by the current of Vin and the choice of R1?
I was wondering why D2 will not always forward voltage drop .6 volts?


Suppose Vout were at +3 V. 

Then if D2 were still forward biased, there would be 3.6 V forward bias on D1, indicating a contradiction in your model. 

What actually happens is if a high enough voltage is applied at Vin, D1 becomes forward biased, with its current also flowing through R1. By Ohm's law, at some point the potential across R1 becomes enough to raise node 'A' above -0.6 V, and D2 is no longer forward biased.