You know, I've been observing something pretty peculiar over the years with this franchise. From a marketing standpoint, the reason Ash's Pikachu has not, and will not evolve is because Pikachu is (still) the whole face of Pokemon, why remove the mascot from the Television show?
Well, that doesn't satisfy me at all, so I've decided to ponder further and further into this and figure out the actual reason. Apparently, the stated reason is that Pikachu "Is a lot stronger than most other Pikachus", but this is actually completely made up by Team Rocket in the second episode. There is absolutely nothing out of the ordinary, or special about Ash's Pikachu. TR seemed to completely miss the fact there were like 15 other Pikachu's at the Pokemon Center, which is why Team Rocket lost the way they did...
But it seems to me that with all this yammering and constant stalking on Team Rockets part, through the years, Pikachu itself has developed the sense that it is "Stronger than most normal Pikachu", in a complete arrogant fashion, and Pikachu does get pretty arrogant starting around the Advance Generation (thank Goodness, no more Togepi!)
So Pikachu arrogantly believes its really powerful, and thus doesn't want to evolve because it would be "Too Powerful."
But you know something? Arrogance isn't the same as Confidence.
Perhaps Pikachu expresses on the outside that to be a Raichu would make it overtly powerful, too powerful that it doesn't need all that power to be "The Best it can be." Thanks to Team Rocket's constant stalking of it, it already feels like its "The Best it can be." but the actuality of it is Pikachu is scared, scared of the new Powers it will possess as a Raichu.
Remember, Pikachu hated Ash when they first met, and they only patched it up because Ash was so goddamn insistent in being friends with it. If Ash was actually a Pokemon Trainer with sense, he would have picked Pikachu up and thrown it onto the Pidgey to capture it, instead of f*cking the whole thing up with those Spearows. (also, WHY DID THE IDIOT ASH NOT CAPTURE ANY OF THE SPEAROWS??)
Pikachu doesn't want to evolve because it DOESN'T want to be the best it can for Ash, because Pikachu still holds the resentment from the Spearow attack, even though Ash did rescue it, the whole situation still broke Pikachu to tears, and it did accept at that point that it will be with Ash forever, but it still never got over the Spearow attack.
A lot of other events during Season 1 sure didn't help to change Pikachu's mind. For the same reason Bulbasaur didn't want to evolve, Pikachu SAW how much of a failure Ash was being with Charmeleon/Charizard (oh wait, how can Ash fail when HE DIDN'T EVEN TRY TO DO ANYTHING BUT MAKE STUPID FACES AT THE DISOBEDIENT POKEMON!!!!????) who too, had arrogance problems but didn't care.
Of course, all of this only applies to the first Season since the writing team has changed many times over the years, but this is what I think.
Well, that doesn't satisfy me at all, so I've decided to ponder further and further into this and figure out the actual reason. Apparently, the stated reason is that Pikachu "Is a lot stronger than most other Pikachus", but this is actually completely made up by Team Rocket in the second episode. There is absolutely nothing out of the ordinary, or special about Ash's Pikachu. TR seemed to completely miss the fact there were like 15 other Pikachu's at the Pokemon Center, which is why Team Rocket lost the way they did...
But it seems to me that with all this yammering and constant stalking on Team Rockets part, through the years, Pikachu itself has developed the sense that it is "Stronger than most normal Pikachu", in a complete arrogant fashion, and Pikachu does get pretty arrogant starting around the Advance Generation (thank Goodness, no more Togepi!)
So Pikachu arrogantly believes its really powerful, and thus doesn't want to evolve because it would be "Too Powerful."
But you know something? Arrogance isn't the same as Confidence.
Perhaps Pikachu expresses on the outside that to be a Raichu would make it overtly powerful, too powerful that it doesn't need all that power to be "The Best it can be." Thanks to Team Rocket's constant stalking of it, it already feels like its "The Best it can be." but the actuality of it is Pikachu is scared, scared of the new Powers it will possess as a Raichu.
Remember, Pikachu hated Ash when they first met, and they only patched it up because Ash was so goddamn insistent in being friends with it. If Ash was actually a Pokemon Trainer with sense, he would have picked Pikachu up and thrown it onto the Pidgey to capture it, instead of f*cking the whole thing up with those Spearows. (also, WHY DID THE IDIOT ASH NOT CAPTURE ANY OF THE SPEAROWS??)
Pikachu doesn't want to evolve because it DOESN'T want to be the best it can for Ash, because Pikachu still holds the resentment from the Spearow attack, even though Ash did rescue it, the whole situation still broke Pikachu to tears, and it did accept at that point that it will be with Ash forever, but it still never got over the Spearow attack.
A lot of other events during Season 1 sure didn't help to change Pikachu's mind. For the same reason Bulbasaur didn't want to evolve, Pikachu SAW how much of a failure Ash was being with Charmeleon/Charizard (oh wait, how can Ash fail when HE DIDN'T EVEN TRY TO DO ANYTHING BUT MAKE STUPID FACES AT THE DISOBEDIENT POKEMON!!!!????) who too, had arrogance problems but didn't care.
Of course, all of this only applies to the first Season since the writing team has changed many times over the years, but this is what I think.