I mean you don't know how many computers you can log on to simply by walking up to the desk and opening the drawer which has a sticky note with the password on it..
My wife works as a doctor at 3 hospitals. Between the three, she has something like 17 passwords to access various computer systems. She also has to change them every 30 to 90 days and she may not even work at two of the hospitals for some months at a time. How is anyone without a photographic memory supposed to keep track of all of those logins? That's not to say my wife writes passwords on a post-it (she'd need her own desk for that), but it explains such behavior. Password policies that require frequent changes result in weak passwords (password1 this month, then password2, etc.)
Disclaimer: "These opinions are my own, though for a small fee they be yours too." -- Dave Haynie