The cover letter is the human side of your marketing collateral. When you are unable to be there in person to shake someone's hand while you give them your résumé, your cover letter is your best first impression. Make your cover letter professional, yet human. Your cover letter should convey a sense of who you are.
It is best to be real here. You don't want to convey an image of stoic professionalism if you are a bubbly and gregarious personality—and vice versa. You're better off not being hired for a position where your personality will create conflict or will not mesh well with the company culture.