Erased my previous comment. It missed the real point.

If you think the doctor should say, "This is the best painkiller I have," that suggests you want to believe you are getting a potent painkiller of some kind. You want to believe that it is a potent painkiller, which is false, as opposed to it is the most potent of the zero painkillers he has, which is true. The fact that the doctor is not technically lying does not change the fact you want to believe something that is false.

If the IV contains a saline solution, the Way may want me to believe the IV contains a saline solution, but I sure as Hell want to think it contains a potent painkiller.

(Yes, I realize the irony in using the expression "sure as Hell.")