This positive resignation letter cites a career change to a new field as the reason for the employee leaving.
April 1, 2012
Dear Mr./Ms. Bossname,
While I have greatly enjoyed my time working for (company,) I am hereby tendering my resignation as (position,) effective (date.) I have long desired to work in the field of (field) and I have finally reached a point where I can move into the career of my choice. I will be taking a position in (company) and am excited for the new direction in my work, although I will miss the relationships I have established here.
I appreciate all of the opportunities you have granted me here and I wish you and the company the best of luck in the future. I will complete any unfinished projects before my departure and I am happy to help train whomever you find as a replacement.