Before starting a PhD, it is important to understand why you are starting a PhD. Here is a list of positive and negative motivations!

Positive motivations for doing a PhD:

1. You have a cutting-edge idea for research you want to pursue, and doing a PhD will allow you to do so. You want to achieve something significant.
2. There is an area of research that attracts you, and you feel you may be able to contribute to it.
3. You want to become an expert in a particular field, with an aim to pursue a related career.
4. You want to discover or learn something new.
5. You enjoy the academic environment and wish to challenge yourself academically. You believe you can do it.
6. You have the opportunity to study for a PhD and want to invest in yourself.
7. You want to develop transferable skills that will help you in your career/life course.

Negative motivations for doing a PhD:

1. Seeing a PhD as a marker of how ‘smart’ you are.
2. Getting a PhD because you want to be recognized or respected.
3. Doing a PhD because somebody else did or wishes they had.
4. Doing a PhD for short-term personal gain.

Source:

The realities of completing a PhD: How to plan for success (Nicholas Rowe)