How does an understanding of sociology help one to understand why people behave the way they do? In what ways is an understanding of sociology frustrating for understanding why people act the way they do?

In simplest terms, sociology is the scientific study of social relationships, culture, and societies. Sociologists study the way that humans within a society interact and behave. Sociology’s subject matter is incredibly diverse, spanning across religion and crime to divisions of race and social class. The study of sociology can reveal shared cultural beliefs or provide a gauge by which to measure stability or change within a society.
When members of a society are faced with a person or situation that is radical or far removed from societal norms, the first question they often ask is, “Why?” You may have seen this firsthand, perhaps in the wake of a particulary disturbing crime. Or, even in observation of societal reactions to a heavily-tattooed person. We may also find ourselves questioning the behavior and decisions of partners in a romantic relationship or business partnership.
Imagine having such a profound understanding of human behavior that you don’t need to question the decisions or actions of others. Understanding trauma’s effects on behavior, for instance, could enable you to empathize with a domestic violence victim who chooses to stay with their abuser, or the troublesome teen who is emotionally closed-off and engaging in high-risk behaviors.
This sort of knowledge is powerful, but can also be frustrating to possess. Simply understanding behavioral motivation is not enough to change the behavior. Though you might know why something is happening, you might not know how to help. You may also be frustrated by the fact that other people around you do not share your knowledge of behavior. You may find their lack of understanding or judgement of others to be a source of aggravation.

Simply put, Sociology is the study of human behavior and interactions within the context of shared experiences, traditions, norms and the way how a group of people choose to spend their time together.
Sociology looks at people as a whole, using certain generalizations which may not be applicable to human beings in a certain society individually. This sometimes makes it difficult to predict human behavior in a group setting. Since human beings cannot be treated as a conventional scientific experiment using emperical data as evidence۔ No two human beings, even though with the same upbringing and life circumstances can behave and react to external factors in exactly identical ways. This makes Sociology a rather uncertain way of understanding people and why they do certain things when together.

Sociology allows us to understand human behavior by using scientific methods to collect data and interpret it in ways that illuminate trends, norms, or deviations in human behavior. This can be done with experiments that mimic potential real life situations, and famously, some sociologists have conducted experiments on human behavior that have had far-reaching implications for sociology. One notable example would be the Milgram experiments that measured the influence of authority figures on subjects' willingness to inflict pain on other people. Subjects were told to inflict electric shocks in increments that slowly increased to other people (in reality those being "shocked" were actors and no harm was done to them). The subjects were merely given neutral orders to do so and were not forced or coerced. The findings indicated that many people only needed very gentle nudging to inflict pain on strangers.
One way in which the findings of such experiments can prove frustrating is the human tendency to disbelieve one's own capacity for cruelty or other negative behaviors. In other words, humans tend to think they are better behaved than they really are, and the findings of experiments such as Milgram's offer unsettling evidence to the contrary. The "big picture" takeaway from such findings is that when human atrocities occur and seem to be the result of one person's authority or ideology (such as Hitler and the Third Reich), we can extrapolate that, in fact, many or most human beings are capable of atrocious behavior under the right circumstances. Milgram's experiment bears this out, even if the results are disturbing and difficult to accept. By accepting the validity of such experiments (assuming they are done with the proper protocols and provide authentic results), we can learn a great deal about human behavior which can then in turn lead to a greater understanding of why we behave as we do.