You're welcome, leeneia.   I was delighted when Maria solved this mystery for me too.

I sing this song very often at nursing homes and retirement communities, because it's such a good sing-along. The melody is catchy and the lyrics are easy to remember, so it's not only requested often but it also gets a lot of participation.
The thing is, my audiences (participants) seem to regard this song, just as they do "You Are My Sunshine," as a happy song, when neither song really is.   Que Sera Sera is very fatalistic, of course. But I guess because the melody sounds light (and you could do a Viennese waltz to it), it kind of feels happy, even though the lyrics aren't.