Here's why:

* DNA contains the genetic instructions for building and maintaining an organism.

* Every cell needs these instructions to function properly.

* DNA is passed down from parent cells to daughter cells during cell division, ensuring all cells have the same genetic blueprint.

Exceptions:

* Red blood cells lose their nucleus and DNA during development, but they are still derived from cells that had DNA.

* Some cells, like sperm and egg cells, contain half the amount of DNA found in other cells.

Therefore, while there are some exceptions and variations, the vast majority of cells in the human body contain DNA.