Almost totally unrelated to chords, notes or diminished intervals! Last time I went to a concert (Weakerthans in April, discussed here and here), I sang along with as many of the songs as I knew, which was most of them. I am almost always singing along with the radio in the car (and sometimes at work, when nobody else is around to be disturbed). I sing in the shower a lot and while I’m just wandering around. In short, I sing a whole bunch.
Apparently this is odd! Most people who were at the concert were not singing. When I play Rock Band, I am often chosen to sing because either my singing friends do not know the songs or my friends who know the songs do not want to sing in front of people. I sing in front of people pretty much bi-weekly (being involved in worship leading at church I sing in front of/with between 30 and 80 people a couple times a month). I am perfectly willing to do karaoke and I have yet to be laughed off the stage so I guess I am good enough at it that people don’t mind.
My theory is this: People who grow up going to churches that are either non-instrumental or otherwise encourage congregational singing (even through choirs) tend to be a) better singers and b) way more comfortable singing around others. There are, of course, exceptions to this that go both ways (I am pretty sure one of them will read this and feel like I am talking to him or her directly but be assured I am not! I can think of others as well!): Some lifelong church people cannot carry a tune in a bucket with three helpers to hold it up with them and there are lots of people who’ve never set foot in a church building who sing very well. It is just a thing I have noticed in my discussions with music-y people.