A class should have only one responsibility

Responsibility is the work that has been assigned to the class. In the SOLID set of principles, the S stands for Single Responsibility Principle (SRP). When applied to a class, SRP states that the class must only work on a single aspect of the feature being implemented. The responsibility of that single aspect should be fully encapsulated within the class. Therefore, you should never apply more than one responsibility to a class.

Let's look at an example to understand why:

public class MultipleResponsibilities() 
{
    public string DecryptString(string text, 
     SecurityAlgorithm algorithm) 
    { 
        // ...implementation... 
    }

    public string EncryptString(string text, 
     SecurityAlgorithm algorithm) 
    { 
        // ...implementation... 
    }

    public string ReadTextFromFile(string filename) 
    { 
        // ...implementation... 
    }

    public string SaveTextToFile(string text, string filename) 
    ...