It is my belief that Pikachu is level 100 or above, actually it is quite possible it is above the level 100th level. Which explains why team rocket is so fixated on this particular pikachu, so early in the anime. Often times this particular pikachu even shows how "high leveled" it is by being able to do things that other pikachus can not do. In episodes 373-374 Pikachu actually shows that it is at a level on par with legendary as Groudon's Blue Ball should only be able to be used with Legendary Pokemon, or at the very least a fully evolved Pokemon, but pikachu was so powerful that he actually went into a primal state. Nintendo has released recently that the new kind of evolution is called Primal Reversion. However in the past all other kinds of evolution's require the Pokemon to be at the pinnacle of its evolutionary state, and this does not seem to be changing for with Primal Reversion either. We all have to remember that the purpose for evolution to begin with is to keep us baited and wanting to know what happens next. So its natural to assume that all other types of evolutionary gimmick will require your Pokemon to be in its final evolutionary stage. I also believe that Pikachu started out at a very high level to begin with. As it was very hard to train, and when it was issued to Ash, it was actually not in a normal pokeball but a Thunder Ball, meaning that people were having a hard time catching it to begin with. I also find it very hard to believe that it was the only Pokemon left, as it was not suppose to be part of the trinity of starters to begin with in the anime, not only that but Professor oak had his lab in the smack middle of pallet town, so he would have a estimate of how many new trainers would be coming in. It is highly possible that the Pikachu was at its Max level when he started with Ash, seeing Pikachu is the result of an experiment conducted by Professor Oak and allegedly destroyed the lab where he was created in a momentary loss of control. It is also this thought that perhaps this experiment not only unlocked the unlimited Leveling feature of this particular pikachu but brought it to a new evolutionary branch, allowing it to surpass its final evolution in both power and ability. This would prevent pikachu from evolving because it is at its final evolution as far as normal evolution. I would also note , that I think the last thing we should remember is that Ash's pikachu is full of emotion. He has good days, he has bad days. He also seems to know when to pull a punch or let it go full throttle. Even the best fighters have bad days when they are in a "meh" mood. I would also like to note that Pikachu has been shown in the anime to be very caring to the other Pokemon. He would purposefully lose just to help the Pokemon's confidence. Someone in the comments mentioned that he lost to snivy, but you also have to remember that Snivy is a very proud Pokemon. He would lose to Snivy just to give it motivation to keep battling and get stronger.