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 )