"You are my sunshine".
For some reason, people seem to think this is a happy song, but only if you only sing that one verse (it is actually the second verse, I think).
Others include:
The other night dear, as I lay sleeping
I dreamed I held you in my arms
But when I awoke, dear, I was mistaken
So I hung my head and I cried.
You told me once, dear, you really loved me
And no one else could come between.
But now you've left me and love another;
You have shattered all of my dreams: