My first novel was rejected umpteen times so I wrote a second one.
However, at least with my first novel, most agents replied. So you at least knew they'd received your manuscript/looked at it.
With my second novel, no agent has even replied. I sent out my manuscript months ago. I've emailed some of them, politely checking if they've received it. Nothing.
It was obviously disheartening that my first novel failed but I understood I needed to improve. I really do believe my second novel is better. I'm not expecting a publishing deal just to fall on my lap but I at least expect some kind of reply. Even just a standard one line rejection so you know to move on.
I think it's rude.