Freud argues that creative writers compose stories that draw inspiration from two sources: fantasies about their career and fantasies that are more erotic in nature. It could be argued that we see both of the elements at work both in the character of Marlow and in Conrad himself.
For Marlow the entire journey to Africa and then in search of Kurtz is one in search of career success. This is Marlow's job and, therefore, he presses on in order to be seen as successful. Throughout the journey we also see subtle suggestions of Marlow's fantasies about the native women as well.
Much more interesting is to consider however how these elements might have driven Conrad to first conceptualize this story. Again, it could be argued that conjuring up a story about a journey into the heart of darkness must reveal something about the nature and the heart of the author or that the art of writing fiction is somehow also a journey into the heart of darkness as well.