In Firefox and Chrome you can see all web passwords in plain text. It used to be the same in Safari but apparently not anymore. And that's when they don't leave their webmail logged in, which a lot of people do and they lend you their computer like nothing happened. This is often a master key to somebody's privacy, usually more critical than the kind of stuff have in their keychain most often (WLAN passwords and the like).

People around me are not the most security conscious, or they just know they can trust me.

I guess there's also the cultural bias to allow people "check their email" and stuff like that.

My keychain is always locked, I don't save sensitive web passwords in browsers, and I still don't let people use my computer unsupervised.