When a Yukkuri sees someone else taking it easier than itself and desires to be as easy as said person.
Yukkuris don't actually get jealous of humans that often as most shithead yukkuris (which get jealous more often than niceheads) look down on humans and believe that it's impossible for a human to take it as easy as themselves. Therefore most cases of jealousy happen when a yukkuri sees another yukkuri taking it easier than itself. The most common example is a stray yukkuri being jealous at a pet yukkuri.
Just like the touhou character they're based on, parsee yukkuris are particularly prone to get jealous