What is the meaning of [Life is not all beer and skittles]

Life is not all fun and games. We have our pleasant time, but we have our
serious ones too. As Trilby says in the book by George Du Maurier that bears
her names: ‘Well, I’ve got to go back to work. Life ain’t all beer and
skittles, and more’s the pity; but what’s the odds, so long as you’re happy. ‘