In Animal Farm, some of the animals are glad to believe that they are better off now, even if they know they are suffering, because this is the nature of denial. This is a human trait—which is one of the examples of the animals quickly transforming into human-like creatures—and is a form of defense mechanism triggered in our psyche when our beliefs are contradicted.
George Orwell has seen the rise of Marxist-based authoritarianism and was an officer in the British Army, thus making him a reluctant enforcer of Great Britain's colonialist policies. This background gave Orwell a firsthand account of the modern era's power struggles and power dynamics.
In Animal Farm, Orwell blatantly criticizes revolutionaries and supporters of authoritarian rulers who turn a blind eye to detrimental conditions created by failed revolutionary programs.
The animals did not become delusional, because they are well aware of the dictatorship, but they force themselves to believe that their sacrifices were not in vain. Thus the animals are forced to choose the new evil regime—one which they feel guilty for helping establish—over the old evil human regime, because they make themselves believe that the former is a lesser evil.