Nobody likes writing a cover letter – it is a minefield. How should you address the person? How formal does the language need to be? Should you use ‘yours faithfully’, ‘your sincerely’, or neither? It’s never an easy task to decide.
One thing you need to do with your covering letter is to clearly demonstrate how you meet the requirements for the position. It might be that you want to expand more on your experience. It could be that you have a real passion for the field and can’t cover everything you want to show in your CV. Make sure a cover letter works in conjunction with your CV to show your personality and how well-rounded you are.
Cover letters generally aren’t informal, but in certain circumstances, it might be acceptable to be less formal. If you are applying for a very informal role that asks you to be chatty, then it might be appropriate to be more casual.