There are countless competing perspectives within sociology. In order to answer this question, we should identify specific fields, or strains of thought, through which to consider the question.
Let us take, for example, the question of economic inequality. From a Marxist perspective, we could argue that capitalism — that is, the notion of private ownership of property, and the exploitation of workers by a capitalist owner class — causes such inequality. We could go further and say that, in our current society, workers have only their labor (that is, their ability to work) to sell, whereas a "capitalist class" would have accumulated wealth (land, invested wealth, and so on). If we wanted to conduct a more thorough, detailed analysis, we could turn to historical materialism, or associated schools of thought.
We could also argue that, from a feminist sociological perspective, that economic inequality is caused in part by the subjugation of women, and the separation of society into public (male) and private (female) spheres. Here, we could draw on Simone de Beauvoir, and other feminist scholars. However, we must recall that feminist thought owes a great deal to Marxist, or broadly leftist, thought; indeed, sociology is a tangled, intersecting web of ideas. Any discussion of feminism, sociology, and economic inequality would have to touch upon Marxism, and its many scholarly descendants.
What if we considered economic inequality from a post-colonial perspective? We could argue that economic inequality has a global antecedent; Western imperialism, and colonial exploitation of the "Third World," is a major cause of economic inequality. We could then delve into the writings of such scholars as Edward Said, Frantz Fanon, and Gayatri Chakravorty Spivak.
If, however, we wanted to explore economic inequality in terms of mundane, or daily interactions, we could turn to the works of Erving Goffman. Goffman argues, in "Stigma," that when individuals depart from socially normative modes of behavior, they are stigmatized; stigma follows them, and may wreak havoc on their lives. For example, someone with a criminal record may be discriminated against in terms of housing, employment, and health care; a mentally ill person who behaves strangely may be cast out of mainstream society, and pushed into homelessness. An LGBTQ person may conceal their identity, or try to "pass," in order to avoid violence. Thus, we could argue that economic equality stems, in part, from stigma — that is, the ways in which stigma excludes certain individuals from the mainstream, and dehumanizes them.