The towel gets “dirty” because you are clean, not sterile, and because a damp towel is a great place for microbes to grow.

What stays on your body

Even after a good shower, your skin still has:

• Dead skin cells that are constantly shedding.
• Natural skin oils, sweat residues, and tiny amounts of remaining soap or products.

When you dry off, all of this transfers to the towel fibers along with the water, so the towel gradually collects that material over multiple uses.

What happens to the towel

Once the towel is wet and hanging in a warm, humid bathroom, it:

• Stays damp long enough for bacteria and fungi (from your skin and from the air) to feed on the trapped skin cells and oils.
• Picks up extra microbes and dust from the environment, adding even more “gunk” over time.

That mix of moisture, skin cells, oils, and microbes is what eventually makes the towel “dirty” and sometimes smelly, which is why it needs regular washing even if you only use it after showering.