Earlier quoted context omitted.
You (or other people) will thank yourself in a few months/years when refactoring the code, knowing that they don't need to worry about missing edge cases, because all known edge cases are covered with these non regression tests.
There's really no situation you wouldn't write a test? Have you not had situations where writing the test would take a lot of effort vs the risk/impact of the bug it's checking for? Your test suite isn't going to be exhaustive anyway so there's always a balance of weighing up what's worth testing. Going overkill with tests can actually get in the way of refactoring as well when a change to the UI or API isn't done be…
Adding tests is not easy, and you can always find excuses to not do that and instead "promise" to do it later, which almost never happens. I have seen enough to know this. Which is why I myself have put more work in writing unit tests, refactoring code and creating test mocks than anyone else in my team.
And I can't tell you how much I appreciate it when I find that this has benefitted me personally -- when I need to write a new test, often I find that I can reuse 80% if not 80% of the test setup and focus on the test body.
After adding the first test, it becomes much easier to add the second and third test. If you don't add the first test or put the effort into making your code actually testable, it's never going to be easy to test anything.