Is it a bad idea to express my personal feelings about the subject when writing email to a professor for PhD application?
To express that you are motivated is  a good thing. The difficulty is getting the "tone" right. This is in part a cultural problem. If you are not German and you happen to know someone who is, then you could ask her or him to read your letter and see if you get the right message across. It would be even better if that person is also from the same discipline, as cultures also tend to differ between disciplines.

In general I would keep this part short and to the point. If you have a specific reason why this is your dream (and it is not too personal) than it might help to mention that.