Your cover letter should be written in a serious, professional tone. If you use anecdotes, you run the risk of not being seriously considered. You should always keep a polite and respectful tone in your letter. Don't sound desperate . Your cover letter should make you sound determined to successfully enter into the industry - not desperate to do so.
Enthusiasm is very different than desperation.
Make sure you don't beg for a position. Demanding statements . You should never demand something from the employer in your cover letter. This insinuates that you expect the employer to place you in a position that satisfies your needs - not theirs. It is more effective to indicate where you feel you could make a contribution to the organization.