Most people are a lot more comfortable and eager to break someone else's code than they are their own.
Not me. I'm just as merciless with my own code as I am with others' when writing tests. Proper testing involves taking on the role of the malicious agent who is actively trying to break the code, feed it bad inputs and generally muck up the works. If the code passes those tests then it stands a much better chance of rolling with the punches in the real world of production.