@inBOIL: To me, the interesting thing about that Len song is that the music consists of a looping interlude of a disco song. Surely disco doesn't have a monopoly on happy music....
@Daniel: I find "Imagine" a bit bittersweet, but I view it as song of hope. It is an example of a song that looks to positive things or possibilities despite the negativity that surrounds us. I view Marley's "Sun Is Shining" as another example of this. It is a bit sad in tone--think of the poverty and racism that surrounded Marley--but it aims to give the listener hope.
Much of the reactions to songs will vary for listeners. What I view as a happy song won't necessarily be happy for everyone. It's subjective and, often, circumstantial.
@little_tippler: Wow, there are so many songs and artists on your list that I don't even recognize. Thanks for putting this together.
May your days be filled with happiness!