User blog comment:Xxace77/Zoro's right eye foeshadow?/@comment-3242083-20110822155240

http://i151.photobucket.com/albums/s126/nevilaro/zoro-sharingan.jpg

But seriously, this is my favorite reason: Zoro gave up his eye voluntarily.

He basically threw his honor away to train under the very man he wanted to surpass. Even if the training itself was hell, in the end he had an "easy ride" to achieve his goal.

So, as a final "gift" for Mihawk he gives him his eye in exchange for the exceptional training - and thus preserving his honor as a swordsman. No one should say he achieved his goal because he happened to be babysat by a Shichibukai.

http://images3.wikia.nocookie.net/__cb20070301181921/onepiece/images/thumb/e/e0/Ouchy.PNG/230px-Ouchy.PNG