Pursuing a career often means getting an advanced degree. People want to live on their own, and often they take classes for an advanced degree while working at a regular job. It takes a great deal of time to get an advanced degree. There are classroom hours to consider. This is true whether a person goes to an actual campus classroom or studies online. After classes are complete, studying, homework and research are often needed to pass classes.
All of this adds up to a great deal of time, and being single makes it easy to accomplish. When a person is single, they do not have to worry about spending time with their significant other. They can spend their evenings studying or taking classes. There are no distractions because the other person wants to talk or go out for the evening. This allows the single person to concentrate fully on getting their degree and advancing their career.