This is the title of a book by Shawn Achor. Here a short quote:

"The Happiness Advantage - Because positive brains have a biological advantage over brains that are neutral or negative, this principle teaches us how to retrain our brains to capitalize on positivity and improve our productivity and performance." p. 17

Is happiness an advantage when it comes to second language acquisition? I believe that it is, or at least something related to it. In writing about the natural approach to language acquisition, Stephen Krashen stresses the importance of something he calls "low affective filter" when working to acquire a language:

"The final principle is that the activities done in the classroom aimed at language acquisition must foster a lowering of the affective filter of the students. Activities in the classroom focus at all times on topics which are interesting and relevant to the students and encourage them to express their ideas, opinions, desires, emotions and feelings. An environment which is conducive to acquisition must be created by the instructor - low anxiety level, good rapport with the teacher, friendly relationship with other students; otherwise acquisition will be impossible. Such an atmosphere is not a luxury but a necessity."

It is a necessity to lower our guard, so to speak. Low affective filter means being open to, receptive, easily moved by what is going on. Language acquisition is shut down when we are bored, stressed, when students don't get along, when the teacher is unhappy with/critical of the students.