I selected Angela "webchick" Byron as my co-maintainer for Drupal 7 back in DrupalCon Szeged in August 2008. Since then, together we shepherded efforts of 1,000 core contributors to create Drupal 7, got the release out the door in January of last year, and worked hard thereafter to stabilize Drupal 7, to the point that the number of Drupal 7 sites eclipsed the number of Drupal 6 sites earlier this year.
However, Angela's level of responsibility in the community has grown significantly in the past 3.5 years, and she wears many hats, from Drupal Association board member to code sprint planner to Drupal.org coordinator to evangelist to general community cat herder. We both felt that it was time to transition the role of Drupal 7 core co-maintainer off of her plate, in order to give her more time to focus on her other community roles.