well, it depends, human is simple, and complicated as well. A man is more than one word.

Along the way in life, we discover things that interest us to try and experience. Things that are new to us beneficial or detrimental. Either way it teaches us to become better. As to answer the main point of the question, people do change suddenly because the milieu they are into won't give them satisfaction. Some would endure to stay but eventually change is inevitable.

People always learn good or bad things from their experience. If the aim of changeable is to be better person, I think people must change.

I think people's inherent, fundamental character does not change, though life-changing events may alter their world view, thus altering their behavioral choices.

Every day the sun is new, every day you and I are new. Nothing is constant.