A comprehensive (132 pages) study from the Journal of Evolutionary Psychology has some very interesting findings:
An Environmental Approach to Positive Emotion: Flowers
Flowers have immediate and long-term effects on emotional reactions, mood, social behaviors and even memory for both males and females.
The study is fascinating and full of interesting passages. Here is one of them:
For more than 5000 years, people have cultivated flowers although there is no known reward for this costly behavior. ... We suggest that cultivated flowers are rewarding because they have evolved to rapidly induce positive emotion in humans, just as other plants have evolved to induce varying behavioral responses in a wide variety of species leading to the dispersal or propagation of the plants.
This is very interesting because it somewhat mirrors the relationship between humans and dogs, something the study addresses later on:
An extensive literature search for research on why certain flowering plants are selected for domestication or propagation yields almost nothing. Many books and articles discuss the domestication of plants useful for humans in food, medicine, shelter and so forth; the notable exception is the domestication of flowers. We suspect this is part of the general neglect of emotional processes as major contributors to biological evolution. Flowers cultivated by humans occur in the wild in disturbed ground. Usually they are weeds taking cultivated land away from edible/burnable/constructive produce. If the flowers induced positive emotions they might have been allowed to remain in or near the cultivated fields. The loss in food production due to weeds would have been offset by an increase in positive emotion. The selected offspring of these pleasing plants might have become even more pleasing. We hypothesize that as flowers moved into the new niche created by agriculture there was an increase in variation and the more pleasing and attracting flowers were allowed to remain. At some point humans might have moved from merely tolerating these weedy species to actively saving and sowing the seeds. It has not escaped our attention that the scenario we present for the evolution of flowers is very much like the scenario that Budiansky (1994) presents for the evolution of dogs. Flowers may be the plant equivalent of companion animals.
Our hypothesis is that cultivated flowers fit into an emotional niche - their sensory properties elicit human positive emotions. The flowering plants are thereby rewarding to humans and in return, the cultivated flowers receive propagation that only humans can provide. Demonstration of such a phenomenon fills several gaps in the literature. It supports the basic significance of emotion for survival.
There are two really interesting things there. The first is that flowers invoke positive emotions, and were in fact bred and cultivated so they would produce the maximum positive emotion. Second is that those positive emotions were so important, even to early humans that had to struggle to survive, that they would devote costly resources (in the from of land and labor) that would otherwise have been devoted to survival (farming, hunting, gathering).