I am currently employed and not happy with my current job as it doesnt provide me new oppertunites to learn. I am in process of wiriting a cover letter for new job.

In that new cover letter, should i mention the reason I am looking for a new job that I am dissatisfied with this current job because it doesnt offer me new oppertunities to learn? Suggestions please. How should i approach this cover letter.

thanks for your time