How to answer "why are you interested in this position?"

I am applying for a position that I'm honestly probably not qualified for, but it's an entry level job in a field I have considered for a long time. The position is a skills trainer at a mental health facility for teenagers. The problem is, I don't know how to say why I'm interested in it. I keep typing things and then deleting them because they just sound fake and contrived. Any advice?

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official