Here's a good game to play on a Friday afternoon when you don't feel like doing any work and are trying to justify that feeling to yourself.
Calculate the number of months that you have been working as a programmer. Call that number C.
Now estimate the number of months that you have spent working on systems that are still live. Call that number L.
Calculate L as a percentage of C. That's the amount of work you've put in that hasn't been wasted.
In my case I make it about 10%. Which means that about 90% of the work I've done over the last 15 years has been wasted. And I call that a bloody good reason not to do any more work this afternoon!