Just like a mountaineer, you will experience moments of pure happiness. When you reach important milestones, receive positive feedback on your work or put together that perfect combination of brilliant ideas. You should however be prepared for some harder times, during which your passion for your topic will wear off, your motivation somehow disappears or the mountain might seem too hard to climb. What will get you through these times?
There are many reasons behind pursuing a Ph.D. Maybe you are looking for the challenge to develop and solve complex problems. Perhaps you really enjoy activities that are related to your dissertation and everyday academic life, such as writing, analysing data and giving presentations. Does the intellectual and international exchange and the academic project management motivate you to top performance? Or perhaps you are motivated by external factors: a PhD as a status symbol, career opportunities linked to the title, or even a higher salary.
Every reason has its justifications, as long as it works for you.