One of the reasons for pursuing a PhD is to gain knowledge. You want to learn new things or have a passion for research. Probably your specific topic of interest requires PhD level investigations and you have the opportunity to earn a degree while doing something you truly enjoy. It could be an extension of your master's thesis; perhaps some research topics you covered during your undergraduate or graduate school courses could raise questions that lead to a PhD topic that you wish to follow up and complete.
Pressure from parents, partners, peers and even lecturers influences the decision to pursue higher education on all levels. The pressure could be positive and reinforcing, such as a case where a university wishes to sponsor a brilliant student with excellent grades. Social pressure may also come from parents who may pressure you to study to the highest level possible because they want to feel that they have a well educated child.
A PhD is the highest degree a university can award you. In most cases, you are highly respected if you achieve this level of education. Moreover, you will earn the title of doctor, which opens several doors of opportunity. At a personal level, the satisfaction of having made a contribution to knowledge in your specific field of expertise can fuel the ambition to pursue a PhD. It also gives people the idea that you are intelligent.
Some people make the decision to pursue PhD studies because of their current situations. A good example is a student whose visa is about to expire because she has finished her period of study. Enrolling a for a PhD course may be a good way to get a visa to remain in the country of study. Other people do not think they are ready for employment and choose the security of the university walls, while some cannot find the job of their dreams immediately after school so they opt to study more.