You cite because SCHOLARSHIP IS A CONVERSATION. (ACRL Information Literacy Frame)
When you write a paper, you join the conversation on your topic. You have researched what others have said about the topic, and you add your own opinions and ideas. In order to fully participate in the conversation, though, you must acknowledge where ideas other than your own come from. In other words, you cite your sources. However, you cannot simply pull a phrase out of a text and cite it in support of your claim. You must understand the context of the statement, and you must be willing to use the citation in the context that the author intended.
While you read a text, think about what the author is saying and to what degree you agree or disagree with their statements. What points would you bring up to dispute claims, answer questions, or support opinions? How is the author interacting with the sources they used to write the text?
Davidson College Library provides an excellent summary of when A CITATION IS REQUIRED in your paper...
...and when you DO NOT NEED TO CITE information.