I would like not to have to keep selecting my chosen font (Ariel) every time I want to write a document with LibreOffice.
If you are talking about Math (which is suggested by the math tag of your question), then use Format → Fonts to define your preferred font settings, and press Default button there.
If that’s Writer, then use Tools → Options → LibreOffice Writer → Basic Fonts to define default fonts for new documents, or create a template with font settings and make it default.
(Ariel must be a beautiful font btw 
)