I test-drive my code. That means, by definition, that each line is covered by at least one test. I know my code works today and tests ensure it still works tomorrow, next week and next year.
If your code isn't tested you may know it works today, after drawn-out manual testing through each scenario. But what about tomorrow? Are you going to do the manual tests for today's work? Come next week are you going to be performing manual tests for this week's work? Of course you're not, you'd never get anything done.
By testing your code you are eliminating the majority of emergent bugs as soon as they happen (you are doing continuous integration aren't you?). This means you produce features faster in the medium to long term as you minimise the number of bugs that arise. A win for business as costs are reduced as well as lead time. I don't know about you but I prefer adding features to chasing bugs too.
If you're not testing, you must enjoy debugging.