Zack's New Bed

I bought Zack a new bed yesterday. His previous bed consisted of 3 "chewed-up" pieces of egg crate mattress that I covered with a sheet from Goodwill. I cover all the dogs' beds with sheets from Bon Wille'.
When the sheets get dirty, I throw them away. Also, the sheets prevent the dogs from chewing and destroying beds. (most of the time)
WHY DO THEY EAT THEIR BEDS???? They love their beds.

HAPPY VETERAN'S DAY