Irving Berlin wrote “All By Myself” in 1921, for “The Music Box Revue” in his then brand new Broadway theater, popularized in the decades to come by Al and Bing and Ethel and Ella. A hundred years later, one verse seems to take on a new context:

All by myself in the mornin’
All by myself in the night
I sit alone with a table and a chair
So unhappy there
Playin’ solitaire.

Looking at all the lyrics, “All By Myself” is clearly a lament by someone lacking love. But let’s not forget that the “1918 flu” lasted until 1920, infecting 500 million people, about a third of the world’s population at the time.
Isn’t it possible that Berlin had that recent world-wide catastrophe in mind, even if subconsciously?
In any case, it seems to fit now, whether sung by Aileen Stanley in 1921 or Ethel Merman in 1968.