The reality of software development—and they don’t tell you this in college—is that writing code makes up at most 5-10% of your time. Design and planning, for me, take up about 25%, testing and debugging probably 25-50%, and the leftover time is pretty much all documenting.*
When I did the original rewrite of the DataMapper docs, it took me over 40 hours of work! I still spend almost equal amounts of time managing the manual as I do the code.
As far as I know, there is no way to make documentation fun. If you like writing, that helps. If you love your project, writing about it is a little exciting, because you get to show off your work. Undocumented code might as well not exist; documented code is there for all the world to appreciate.
Maybe if you think of it that way it helps.
* Numbers in the post are completely made up. Your numbers may vary based on actual time spent. Project and team size can be a factor in determining your time usage. Results not guaranteed.