Here is what happens the "very first" time the wave starts from the generator ...

[![enter image description here][1]][1]

Write that the Voltages of incident wave + reflected wave = voltage of transmitted wave : \$Wi+Wr = W12= 1 + (Z2-Z1)/(Z2+Z1)\$      
\$=(Z2+Z1+Z2-Z1)/(Z2+Z1)\$    \$=2*Z2/(Z2+Z1)\$.


And when the wave come from the other side ...

[![enter image description here][2]][2]
 


  [1]: https://i.sstatic.net/NnVY5.png
  [2]: https://i.sstatic.net/n4JyQ.png