I think these smells hit so hard because it is so rare that all the right conditions meet in order for us to smell them so now in autumn there have to be the right temperature, pollen release in the air and no wind it makes for the perfect combination for us to experience these nostalgic smells.
I especially love the summer smell it´s pretty rare where I live to experience the one I talk about, again where all the right conditions meet but in the summer it has to be very hot to the point where we have tropical nights, with the right amount of pollen in the air, no wind, pretty high humidity too I think and it´s not even every summer I experience this perfect cocktail, it just smells like childhood.
Although some of these smells are rare I feel every summer the smell of certain flowers that hit so hard I stop up because the nostalgia hit me like this below.