[1457389]

Sadness is also linked to creativity: http://psychcentral.com/blog/archives/2015/08/27/happy-sadne...

Bunch of stuff out there to support it, anecdotally I would agree. If you're too happy why bother doing/thinking differently?

[threatofrain]

When you're frustrated you also behave more variably, and behavioral variability is important to the discovery of novelty.