Remember the doubling shilling from The Time Machine — money growing on top of money? Debt is the exact same machine, running in reverse, against you.
When you do not fully pay a loan, the interest you missed gets added to what you owe. Then next time you are charged interest on that bigger amount too. Your debt starts growing on its own — the same way savings do — except now the snowball is rolling downhill straight at you, and you have to outrun it.
That is how a small loan someone forgot about becomes a monster. It was not the original amount that hurt — it was leaving it to compound.
The lesson is not "fear all debt." It is: before you step in front of any loan, know exactly how fast its snowball is moving.