Why are millennials so open about everything? They assume nothing is private, I guess, and so disclose everything to anyone.

Generational. Millennials in general are seen as open, needing feedback, and favor work/life balance. The roots of this are partly based on how they were brought up (by baby boomers, who encouraged sharing) and partly based on the easy and increased sharing of social media. Modeling and teaching appropriate boundaries can help when sharing is too much, especially in the workplace.
Different generation. Different generations have different perspectives and values.
Generation y? They are immersed in the information age constantly texting blogging tweeting etc & the cyber world doesn't seen real orseem2 count--some times the consequences lead 2 real life problems however.