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. When the mountain will seem just too high to climb, your passion for your topic will wear off, and your motivation somehow disappear. What will get you through these times?
There are many reasons behind pursuing a doctoral degree. Maybe you are looking for the challenge to develop and solve complex problems. Perhaps you really enjoy activities that are related to your thesis 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 Ph.D. 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.