I talk a lot when I'm around people I know well, or am in a conversation that really interests me. Other than that I'm pretty quiet.
When I don't talk, it probably breaks down like this:
40% Can't get anyone to listen (I try to talk and people start talking over me)
35% Afraid of rejection
20% Nothing to say
5% Can't find the right words
The first two might be reversed though, depending on the situation.
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"I don't even know how to explain it, but this is not my dimension, and my mind is never at peace; it's always somewhere else." - Josh Groban, Alla Luce Del Sole