There are a few reasons.
I have spent my entire life since age 7 involved in the martial arts at various points in my life. As such, it has become ingrained in me to be constantly improving myself, especially in ways where I can see the results myself.
Before I had discovered my ability with writing, I had mostly seen this improvement in physical aspects. However, when I started writing, I started seeing a more intangible but equally powerful area of my skills improving.
The thing that made me throw in entirely to writing was when money became tight and I no longer had the ability to shell out $100 a month on my martial arts classes. But, with constant improvement still being an important part of my lifestyle, I had to fulfill that drive and now writing was the only outlet for it. (And of course, it helps that I receive a small stipend from my writing as well that is now starting to approach the same amount I used to have to pay for my classes.)
I have gone to the obsessive effort of doing what I can to improve every conceivable aspect of my writing. It has been a truly amazing journey from knowing almost nothing about literature and being actually quite bad at writing (my first earliest works I posted to SH are after 8 years of practice on works that were never published, and I really sucked back then. Even my first SH postings suck compared to what I can do now.) And now, I would in no way call myself anything truly amazing, but I am improving at an exponential rate both in my own ability and also in my knowledge base of literature-related information. This rate of improvement has pushed me to a point where I can at least look to the real greats and call them a reasonable goal. A goal which I plan to pursue with a fiery passion.