It’s complicated and most people do not understand what the debt ceiling actually is.
Robert Smith of NPR says it’s like this:
The way I put it is that Congress has already ordered the pizza. They approved the pepperoni. They called up and had someone deliver it … Now the pizza guy is knocking at the door, and asking to get paid. If you don’t raise the debt ceiling, it’s like saying we didn’t want that pizza in the first place. Maybe he’ll go away if we don’t answer.
If you’d like to understand a little bit more, read 7 mistakes journalists make when covering the debt ceiling debate (& how to avoid them), a rather painless and relatively brief explanation.