"Just as the wise accepts gold after testing it by heating, cutting and rubbing it, so are my words to be accepted after examining them, but not out of respect for me."
– Buddha
I love this quote by the Buddha. Within the software world, it translates perfectly into the healthy habit of never trusting code just because someone smart wrote it or because it's been working fine for a long a time. If it has not been tested, code is not to be trusted.
Why are tests so important? Well, for one, they give you predictability. Or, at least, they help you achieve high predictability. Unfortunately, there is always some bug that sneaks into the code. But we definitely want our code to be as predictable as possible. What we don't want is to have a surprise, in other words, our code behaving in an unpredictable way. Would you be happy to know that the software that checks on the sensors of the plane that is taking you on your holidays sometimes...