Edit: @kellymce pointed out this is another good example (and a much better one, I think!):
Students can sniff out a try-hard. Sometimes I'm tempted to share, particularly when I'm working with student retention-related groups, that I dropped out of college for awhile and also hated using the library and didn't care to ever learn. But I don't! Because I'm supposed to be the authority in the class and I'm in charge. Can you imagine if you went to a therapist and they started telling you about all their psychological issues? Their credibility would be shot, and it would also be very confusing as to why they are sharing this information. As instructors, we are there to teach a particular subject and guide students to learning. We can relate to them in small ways, in a mentor-ish capacity, but emptying out the closet skeletons is not an effective way to motivate or draw students into learning.
Anyhow, these are my thoughts and I realize how strong they are after reading this article that came out today on Inside Higher Ed: TMI from Professors (study indicates role of over-sharing by professors in encouraging uncivil student behavior). Apparently, students are less likely to behave well in class if you try and rap with them (as in the outdated 70s slang for talk/relate to). Check it out, interesting stuff.
"When students reported that their instructors engaged in a lot of sharing about their lives -- particularly stories about past academic mistakes, even stories designed to stress that everyone has difficulty learning some topics -- there is an immediate and negative impact on classroom attitudes."