Before one can understand the madness, one first has to recognise it, although it has been made very difficult by the social and political taboos which evoke accusations of "xenophobia" and "racism" against anyone who does and dares to say so publically.
I'm sure that many people do recognise this madness, at least from time to time, but dare not say so, certainly not in public, for fear of being branded a "xenophobe" or "racist"; and because it cannot be expressed publically there can be no shared debate, testing or evaluation of this perception, which we are thus left alone to doubt and, in the face of such overwhelming social pressures, eventually reject the validity of, bravely seeking to suppress our own "xenophobic" and "racist" tendencies.
The human brain is such that it can be prevented from recognising even the most obvious things if the taboo against doing so is strong enough. Such taboos can be put in place artificially by post-hypnotic suggestion, for example, or, more naturally, by social and political conditioning, through a regime of rewards and punishments, or promises and threats thereof. Just as we train and condition dogs, so we are trained and conditioned ourselves, to be obedient subjects of the state, as well as a "human resource" and consumer for the economy.
The state wants mass immigration and a multi-ethnic society, and thus trains and conditions us, its subjects, to accept it. And if that fails, we are simply bullied into submission by being branded as "racists", just as in the Middle Ages people were trained and conditioned to accept church dogma (and authority), and those who resisted were bullied into submission by accusations of being "heathens", "infidels" or "heretics".
Mass immigration is madness because our country and subcontinent are already, natively and unsustainably, overpopulated. Mercifully, Europe's native population has peaked and is now slowly declining, which, for sustainability's sake, is exactly what needed to happen. To reverse this natural trend, by encouraging people to have more children and through mass immigration of people's with a higher fertility rate, is utter insanity. It beggars belief that our politicians could be so stupid.
The creation of a multi-ethnic (multi-racial and multicultural) society, which the madness of mass immigration has given rise to, is madness in its own right, because it is difficult enough for people with everything in common to organize themselves harmoniously and get along together. The less we have in common, the more difficult (if not impossible) it becomes.
Which brings us to the method, i.e. purpose, behind this madness and thus to understanding it: the state and capital do not want us organizing ourselves, but as has always been their way, want to keep us dependent on them and do the organizing for us, thus facilitating our self-exploitation as a human environment and resource.
State and capital are so overwhelmingly powerful, and we so utterly dependent on them, that it is difficult to imagine how we can possibly escape their clutches. But if we want our civilization to survive, that is what we must do.