In order to answer this, we must first understand what modernization is. Modernization, in social sciences terminology, is the process by which a country moves from having a traditional, agrarian, rural society to having a more secular, urbanized, industrialized (or even post-industrial) society. When this happens, the country changes in many ways. It changes in terms of its values and beliefs, moving from traditional beliefs to more scientific and secular beliefs. It changes in its geography, with its population become more and more urban. It changes economically, moving from subsistence farming or something close to it to an economy in which hardly anyone farms and the country makes its money producing goods and services.
There are at least two major and related causes of modernization. One is the growth in science that came along (in Europe) with the Enlightenment. The other is the growth in available technology. When a country is pre-modern, its people generally do not believe in science. They hold traditional beliefs that typically hold that life is affected by supernatural forces. They do not believe that life can change in any major ways. In the Enlightenment, people came to challenge this idea. They came to believe that life is affected by forces that are understandable via science. For example, they came to believe that disease is caused by microorganisms, not by demons. Once they had this outlook, they came to believe that life could be improved. Science could change the way we live, making us (for example) less afraid of disease or of other natural calamities.
Science, of course, helps people to develop technology. Technology pushes modernization along. Technology allows farmers to grow more food, freeing people to go live in cities. Technology allows factories to arise, thus giving the new city dwellers jobs. Technology also improves life, giving us things to do for entertainment, things that can improve our health, and many other beneficial innovations. These things lead our society to become more modernized.
Modernization has its good and bad points. On the good side, modernization improves our lives in many tangible ways. It is certainly safer to live in a world in which we have doctors who can cure infections and in which women are not very likely to die in childbirth. It is more convenient to live in a world where we have access to many material goods to make our lives more comfortable and more fun. Modernization also helps people who are oppressed in traditional society. Modernization allows women to have more opportunities. It helps to liberate oppressed minority groups. In these ways, modernization seems like a very positive thing.
On the other hand, it is possible to argue that modernization has negative effects. One negative effect is on our environment. Modernization brings technology that consumes energy and leads to such things as air pollution and climate change. Another negative effect is (arguably) on our society. Modernization breaks up the social ties that bound people together in traditional societies. It makes it so that people no longer feel as connected to one another. This can lead to such problems as crime and the break-up of family groups.
Thus, modernization has clear causes and it has effects that can be positive and negative.