Sometimes I like to rely on gut-feelings while coding.

Ever filled in that user-signup form for the thousandth time?

Ever start with nice input: John Doe, 555 fake St

Then after the 500th time you are just inputing:

aaa abc wsdfg

And clicking "sign up"?

That is the point you should know in your heart: "I missed writing an automated test somewhere".

If you can't relate to the above experience (and have never had your code broken by a coworker), then maybe for some reason you are in a position where you're fine without TDD.