Why Are You Interested In This Position? - Asap Please

Hello Ladies,

I am applying for a junior level position with the client Retirement department. As a process of submission I need to fill out few questionnaires and I am stuck at this typical/general question "Why are you interested in this position?". I have entered a casual answer saying "I am interested because I liked the job description and skills that I can contribute my knowledge". But, I know it could be better and I am not satisfied with my answer and not exactly know what to put an answer in a ways it should make sense and my application can be short listed for final interview. As it is a junior level position, I may be little over qualified (not sure how they consider), because of that I don't know how can I add more saying I can learn this or that other than the domain knowledge.

Please pour your thoughts and what would you say to this question if it's you?