probably every time I see my tests failing.

Each time I am writing tests I'm convincing myself "it's an investment", "spend 2 hours now to save 2 days later", "unit-tests are good".

And each time I'm chasing away ideas like "perhaps they are right, perhaps writing unit tests is a waste of time..", "this code is simple, it should ever break - why test it??", "In the 2 hours I'll spend writing those UT I could build another feature"

Yes, it is terribly annoying to write tests, especially after writing the production code (code-first approach). Why test code that you know works, right?

But after a few weeks, months or years, when the time comes to change your feature: enhance it, refactor it, build an integration with/from it, etc, I feel like a child who found a forgotten favourite candy in his pocket when I see my tests failing.

It means I did a very good job writing them
It means it was not a waste of time
it means these tests will now save me hours or days of trial-and-error change→compile→deploy→test cycles.

So yeah, whenever I see my tests fail, I feel warm and fussy inside :)