Not sure if its that perse, although it seems that Yukkuri have a limited capacity to understand things (they have the child like thing of thinking everything works by the same perspective of theirs, meaning that they think that the sun is tormenting them, when its just hot, since they don't understand the idea of the sun just being a ball of flame).
The Reimu is just lamenting the loss of her child. Its been so mild here in Californa, its hard to remember that if a heat wave is bad enough people die.(Really young and Really Old mostly.) Just last week there were sightings of sunbathers in Moscow.
Maybe if Yukkuris tried to understand better the world instead of using childish logic to blame the uneasy to everything else, some of their problems would had been solved.
I mean, they really don't try to understand, and it's a dangerous way of living since, through understanding you may develop countermeasures or get ready for the "uneasy hot season", specially considering that they understand the concepts of winter and needing to remain in home since freezing is "uneasy".