Why didn't Pikachu evolve? I'm talking about Ash's Pikachu: obviously he already reached level 314159265kachu (Pun intended; that's not the exact number as I don't know what level he is). 

I forget the reason for this - did he not want to evolve? If so, then why did he refuse to evolve? And does that mean that evolving is a choice? You can choose NOT to evolve your pokemon?
I forget the reason about this, did he don't want to evolve?


It's pretty safe to guess that part of the selling point of the series is that Pikachu is "cute". If you let him evolve into Raichu, you've lost this appeal.

So from the author's standpoint, there is no reason to evolve Pikachu. In the Anime, it was expressed as Pikachu simply not wanting to evolve. (Somewhere in the episode where Ash takes on Lt. Surge.)


  And is that mean evolve is an option? You can choose NOT to evolve
  your pokemon?


In the games, evolution is almost always optional. In the Anime, it's a bit unclear.But in this case Pikachu evolves by a Thunder Stone - which is completely optional. If you don't want to evolve, just don't use the stone.