I consider myself a courteous cold caller. I never call a number twice in one day, I rarely call 2 days in a row, avoid leaving more than one voicemail per week if that.
That being said, when someone finally does answer and are immediately hostile it's almost like I'm being tickled and can't hold back the loud laugh. Personally I find it ridiculous that someone would answer a phone screaming when we have never spoken and it's not like I've blown up their phone, a simple not interested would suffice.
I think my laughing is a way to piss them off before I mark DNC...
Oh, I forgot to mention that I promptly hang up on them after the laugh so it's all they hear.