I think it's the anticipation. My ex used to like to get me with them but she'd always announce it first. For example we'd be watching TV and I'd ask if she wanted a cup of tea and she'd point at my stomach and say yes but first I had to let her give me a raspberry.
I don't know why but I'd start laughing right away and she'd very calmly tell me to lift my shirt up and lie down for her. She'd really drag it out to build anticipation. I remember as she leaned down her hair would brush my skin first, then I'd feel her breath, then her moist lips would seal over my belly button and then she'd start blowing. Eventually there'd be some nom-noms and then a big wet kiss when she was done. And then she'd let me go and make the tea. I think the anticipation was the worst part though.
To be fair, for every raspberry I received, I think I gave her at least 20 in return and I'm sure she'd argue that's a low ball estimate.