Combat sports, almost by their very nature, usually doesn't lead to a lot of happy career endings.
In most cases, fighters retire either due to injuries piling up, and the reality of losing fights that perhaps at another point in time they feel they would have won. Often it's a choice made for them. And most of the time, the last fight is anything but a career highlight.
For Mark Munoz, that easily could have been him, losing in just 1:40 at UFC 184 to Roan Carneiro, an opponent who by reputation was hardly as tough as a multitude of opponents Munoz beat during his career.