[derefr]

Strawberries contain serotonin! This is pretty well-studied; it's hypothesized they do it because serotonin in the gut acts as a laxative, causing you to expel the strawberry seeds without digesting them.

If the serotonin manages to make it into your bloodstream (e.g. if your gut bacteria don't want it, and your stomach pH isn't low enough to destroy it, and your gut permeability is high) then you'll certainly get a mood boost.