Automated testing tools exist for a reason. When it comes to testing the work that we've created manually, you'll know from experience that this is a long, (sometimes complex) process that does not allow for consistent results. Not only do we have to manually remember whether a particular component works (or otherwise write the results down somewhere!), but it isn't resilient to change.
Here are some phrases I've heard over the years when testing has been brought up:
"But Paul, if I write tests for my application it'll take three times as long!"
"I don't know how to write tests..."
"That's not my job!"
...and a variety of others.
The point is that testing is a skill in the same sense that development is a skill. You may not be immediately great at one or the other, but with time, practice, and perseverance...
