[lemursage]

It's not a statement about your general motivation for a PhD -- I don't think you can easily find a subject for which you will find a non-stop motivation 24h/7days for 3-4 years (or so, eqv. period required to get a PhD). If you encounter a dip in interest, which most likely will happen, you need to understand that changing a PhD is not always a best option -- just because it sounds good now, it doesn't mean it will in 2 years time. You supervisor should be there to guide you into interesting areas of research and may play a substantial role in rekindling your motivation. Naturally, mental health issues are another problem -- but I think they have often different origins than "just boredom" with subject. I may be completely wrong though.