I wrote an article on one reason technical divers are motivated like this five years ago.Everyone has their own motivations. Some simply like to go places where few have ever gone before. Some like peace and serenity found in the deep water. Some like the technical challenges of pushing themselves.
A short summary is that for certain people, there is an inherent joy in performing a task well when you know only a tiny percentage of the population can do it at all. The ability to perform at that level is the result of many years of dedicated training, and for such people, that dedicated training itself can bring that joy.
I recently interviewed a diver who had risked his life in performing an unpaid service to others (I won't be more specific because I don't want to get into the specifics of individual, who is well known). I am quite sure that in providing that benefit for altruistic reasons, deep down inside he felt a joy in knowing he was doing something few people on Earth could do. Some people would think less of people for having such feelings, but it does not bother me a bit. If you read this more scientific article on it, you will see that it is a character trait of many of the most accomplished people in all fields of endeavor.