Social and political change can be caused by legislation, but more often legislation is gradually adapted to social change caused by other factors. While some of the changes over the past few centuries have been reflected in amendments to the constitution, others are not legislative in nature.
The first major change in the US was westward expansion and the end of the frontier. Subsequently, increasing urbanization changed the population distribution and lifestyles of people in the US.
The intertwined elements of technology and economics have resulted in substantial changes in people's lives. Trains and then cars created vast, sprawling conurbations, and lifestyles involving commuting and suburban living. Technology has dramatically changed people's lives. In the 18th century most people were involved in agricultural labor; now farming counts for only 1 percent of the US GDP. Improvements in medical technology mean that most people live longer and healthier lives.
One of the greatest changes in the United States, reflected in the 14th Amendment, was the elimination of slavery, and the ensuing Supreme Court decisions desegregating schools. The mechanisms for this included a civil war, a constitutional amendment, protests movements, legislation, and a general shift in attitudes.
The next major cultural change to the United States is due to successive waves of immigration, leading to a culturally and ethnically diverse society. Demographers project that white non-Hispanics will constitute less than half the US population in 2045.
Another major change is in attitudes toward and the legal status of homosexuality. In the 18th and 19th centuries, "sodomy" was illegal. In the 21st century, increasing numbers of states are performing gay marriages, and protecting the rights of gay people; it is likely that this will become universal over the next few decades. The mechanisms for change have been court decisions, protests, media coverage, state level laws, and attitude changes.
Overall, changes in people's lives in the United States occur due to economic and cultural change, demographic shifts, cultural change, municipal and federal laws and legal decisions, and technological change.