There is a level of pain which can turn a person into a dumb, suffering animal. In fact there is more than one such level, approached in different ways. A person can be worn down by pain into a dull, unresponsive lump - or suddenly transformed by pain into a writhing, stiff body which can only make incoherent sounds. A person was there, and then just gone. When and if the pain again recedes, a person may again inhabit that animal- but it might not be the same person.
Maybe you don't believe that this is true in general, or that it can't happen to you. It's difficult for a person, or for a personality, to imagine one's own annihilation. I have experienced this level of suffering at first-hand, and have also observed it in others.
It's not a binary phenomenon. Smaller pains can alter one's personality to a smaller extent. If you have ever spent time around someone who suffers chronic pain, or if you have suffered from chronic pain while monitoring your own behavior, you will have noticed personality changes, sometimes drastic ones.
Pain isn't the only thing which can alter one's personality (and by "pain" I mean more than just physical pain: suffering in general is personality-altering). Some personal changes affect the person (and the animal in which the person inheres) positively, and some negatively. Some changes conduce to the development of personality, and some to its disruption.
Your personality, any human personality, isn't real in the way that stones or houses or Anthropogenic Global Warming are real. A personality is real in the same sort of way that Granny Weatherwax is real. Granny Weatherwax is a fictional character in a series of fantasy novels by Terry Pratchett. She appears in many of the stories and is alluded to in many others. She is an established character who, while capable of occasionally surprising the reader, can nonetheless be depended upon to act in certain ways in certain situations.
You are a fictional character in the stories you tell yourself (and others) about the world. Your personality is the character you have chosen to inhabit. It is a work of art informed by itself in that your personality determines your ideas about what your personality should be.
Your personality is also the interface which others use to model your behavior and to determine how they will act toward you. Without personality you are just a dumb animal. The use of language implies personality: an entity without personality has nothing to say. This means that human personalities are necessary for social function. Without personality, animals can't model one anothers' behavior, and without that modeling there is no basis for interaction. If you can't predict the actions of another then there is no point in risking interaction; no common ground can be found except by accident, and all cases of resource contention can only be resolved by the most naive solution: conflict.
Some social institutions make a practice of 'tearing down' personalities in order to 'build them back up' into a more institutionally-acceptable shape. This is usually accomplished through hazing and indoctrination. This is a form of remedial social engineering in that the personalities being 'broken down' were formed by other social institutions in the first place: by families, neighborhoods, communities and so on. This can lead to difficulties if and when the people who have been 'reformed' by the former institutions return to the latter context. Remedial measures such as this are an indication of poor design, or more probably a lack of design.
The process of 'breaking down' personalities is a dangerous one in cases where the results are unpredictable. An institutional 'reform' process designed to produce tractable subjects of authority instead can produce resentment, rebellion and resistance to that authority. This can produce a wasteful cycle in which the institution escalates the abuse which is intended to produce the intended effect, while instead increasing resistance to itself. Even if the proportion of rebellious elements produced under this escalation is the same as or less than that which occurs prior to escalation, the overall numnber of rebellious elements increases, making institutional disruption more likely. In as much as social interactions depend on consent and consensus (interactions which don't depend on these things are called "fights"), these cycles erode the legitimacy of the institution within the larger context of society.
The case of unpredictable results from personality breakdown is the common case. There is an entire, burgeoning industry dedicated to repairing the personal and interpersonal breakage caused by such breakdown. If a sufficiently-sophisticated science of personality existed that it were possible to engineer the breakdown and rebuilding of personalities, it would not be necessary to so so since such engineering could be accomplished in situ without the imposition of undue suffering upon the subject.
The aim of institutional hazing and suffering aside, the effect is the destruction of personality. An animal without personality is a thing. Turning people into things is the beginning of evil. Granny would not approve.