In order to talk about why cultures change, we first have to have some idea as to what culture is. Let us look at the definition of culture from the link below. It says that
In social science, culture is all that in human society which is socially rather than biologically transmitted...
This means that the term "culture" covers pretty much everything about our society -- what religion, if any, people follow; what music they like; how they are educated.
When we look at things in this way, cultural change can occur for any number of reasons. The foods that we eat in a culture can change when immigrants come to the country and bring their cuisine with them. The music we listen to can change in that same way. Education can change when there are preceived threats to our country -- this happened when science and math education boomed after the Russians launched Sputnik.
Since culture is such a broad thing, many things can change culture. However, I would argue that the most likely cause of cultural change is contact with other cultures whether that be through immigration, through media (seeing other cultures on TV, etc) or other types of contact.
I also think that even if a county or just a certain area's population stays about the same, that the culture still shows some change over time. I think it has a lot to do with survival. People have to adapt to the current political changes, economic changes, environmental changes, and other changes that occur over time.
All of these changes lead to people redefining certain traditions, or eliminating them altogether, and doing what they need to do in order to survive. This all leads to a new way of life being more acceptable because people can see a need or justification for it.
As one example, take women working outside the home. It wasn't even very long ago where men were the primary income earners.
Things have changed drastically in recent decades. Not only are many women working outside the home, they absolutely have to in order to survive, as a 2 income household is so often needed with the high cost of living. Staying at home is not even usually encouraged anymore, and girls are told from a young age to seek careers.
Culture of course has to be recognised as a term that is not static or discrete but an entity that is constantly evolving and changing, reflecting the massive changes that impact the world. Recently of course, particular interest has been paid to this term as it has been recognised that the process of cultural change has only been accelerated by globalisation, which means that other cultures impinge on our own culture, mixing together and forming hybrid blends. One key ingredient of this process is of course immigration, which has increased massively in recent years.
Pohnpei has quite adequately defined culture. Countries and their populations change. Immigrants move in, and immigrants move out. This creates a culture in a state of flux. The mixing and meshing of these different ideas creates change. In addition, people adapt their culture as their lives, dreams, and aspirations change.
I think that one reason culture changes is that the population of a country or area of a country changes. Different cultures bring their traditions and beliefs with them. The media also plays a role in culture change, the way they present different cultures has an influence on us also.
Culture change occurs for a variety of reasons. As technology and people evolve, so does culture. Nowadays many culture changes are technology based. For example, in the United States, culture has changed from a well-speaking social country to, at least for a lot of young people, an anti-social, technology-based country.