I won't lie to you. As a brand-new PI, it was kind of exhilarating the first time a student asked me to be on their thesis committee. But the shine has definitely worn off. Because you know what? Apparently I am damn popular. I believe this is the "new meat" phenomenon. Everyone and their dog wants me to sit on their committee. And it is too fucking easy to say "sure I'll be on your committee". I don't want their boss (who will be voting on my tenure) to feel like I am dissing them. And then, next thing you know, you are on like 15 thesis committees*. Which doesn't seem so horrible until everyone wants to schedule their general exam for the same week in October. FML.
SO, new rule: I will only be on 2 new thesis committees this year (I have two grad students). It is too arbitrary for me to decide which students I say yes to on a case-by-case basis. I'm hoping that by setting a hard limit it will be easier for me to say no. And then I will have to come up with a reasonable number that will be my limit in upcoming years. I don't know what the "correct" number is. I just know that I am WAY past it.
*this is, sadly, not really an exaggeration :-/