Ash doesnt believe in evolving his pokemon to the next level! No its the worst thing ever to him! To let his pokemon evolve (which is natural) means hes losing a friend! What just because you haven't mature since you left pallet town means you make sure your pokemon don't mature and stay at their basic stage? Ever notice when his pokemon evolve how sad he gets? jeez to me my pokemon are my children I want them grow up and want them to reach their max potential not hold them back.
Ash doesn't get sad when one of his Pokemon evolves. He's psyched as hell when it happens. The whole 'Ash never evolves his Pokemon' thing has merit, because he rarely does, but not because he's personally opposed to it: If his Pokemon don't want to evolve, he doesn't force them to. That's it. End of story. He gives them a choice whether or not they want to evolve, with all the power associated with that, or stay in the forms they personally like.
He doesn't push them to do it, he trains them as best he can, works with them, if they want to evolve, and do, then he's really excited and proud. If they just get stronger without evolving, that's cool too.