I think the only time I got a little annoyed, someone wanted to do an art trade me and then flaked out after I took them up on their offer. It's not really about them I just got excited and then got disappointed and confused when they never responded. I wasn't sure if I should start on my side of the trade or not. Turns out I was right since it's been years since then.
I rarely send PMs though. Usually I send a VM if I want to start a convo. I rarely have anything to say that requires the privacy or length of a PM.