be another of life's great mysteries

To be difficult to understand or decipher. This phrase is often humorously applied to unimportant things. The living cannot truly understand death—it's another of life's great mysteries. Why dogs and cats hate each other is another of life's great mysteries. A: "Why is our two-year-old screaming about his foot being cold after he just took off his shoe himself?" B: "It's just another of life's great mysteries, I suppose."