I've only talked to some people over Skype and Google + but, I have had some experience with Spanish, if it might help.
What prompted me to speak Spanish "in the wild:" Being in Mexico and being the only person in the group who spoke Spanish; wanting to join in with the conversation when friends were speaking Spanish; being in a situation with other learners and wanting to practice; friends who were taking French would speak French so I'd speak Spanish back; wanting to communicate.
What made me think that another day/time/circumstance would be better: People who spoke both English and Spanish but who preferred to speak English in mixed company; people who were learning English; someone else had a better command of the language so I didn't need to translate or anything; general embarrassment and a fear that my aunt would gush too much

(you know how that is!); feeling guilty for, maybe, being perceived as "showing off."
With me, the most weighty cause for speaking the language was "necessity," which I'm using very broadly here. Obviously, being the only person in a group who can speak makes speaking a necessity. But, wanting to be a part of the conversation isn't, in the strictest sense, necessary. It's necessary to me to be a part of my friends' group, though, in the social sense. When I feel like an outsider, as when the Spanish speakers preferred to use English when a native English speaker was present or, when someone else had a better command of the language, I would defer.
Also, I was much younger and a lot shier, so that had something to do with it - social conventions, not "tooting one's own horn," not "showing off," not "one-upping" other people. In those situations, it would have been helpful to me to have people insist, in a friendly way, that I join in with the language, maybe just include me without making too much of a fuss (maybe, knowing I was learning, just assume that I could follow), rather than allow me to shrink into the wallpaper.