Isn't the main impulse behind pushing the button simple curiosity rather than boredom? I'd be sitting there thinking about what the shock would feel like until I gave into my own curiosity and pushed the button. However, I still definitely like spending time on my own with own thoughts.
I thought this too at first, but it turns out they exposed everyone to the shock along with a bunch of other stimuli in preparation for this part of the experiment. Also one guy shocked himself 190 times.