I hate having somebody's password because if something happens the blame could fall on me too. But unfortunately lots of passwords get in my hand as I work as a programmer... But when someone gives me his/her password, even if they are my clients or my friends, I ask them to change it after I finish my work.
This avoids temptations too
I could just say that I wouldn't like to have somebody checking my profiles or messages or any other private stuff, so I'd rather not do this to someone else...