To write secret admirer notes, you should ensure that you stick to what you know.
Sometimes, people make the mistake of writing stuff that they didn’t come up with originally. They may get quotes from famous poets or writings from obscure philosophers.
Anything taken, that is essentially not of your creation, is a bad idea. It makes your secret admirer notes appear fake and forced.
Even if you think that there is no way this person you admire will know, think otherwise.