There are men so philosophical that they can see humor in their own toothaches. But there has never lived a man so philosophical that he could see the toothache in his own humor.

This quote means that while some people can be so philosophical that they are able to find humour, even in difficult and unpleasant situations (like having a toothache), nobody is so philosophical that they can find humour in their own jokes. In other words, no matter how philosophical someone is, they aren’t likely to find the funny side of their own sense of humour.