When you work out, your brain is focused on regulating things like your limbic system (emotions, motivation) and coordination, explains study author Brent Coker, Ph.D., consumer psychologist at the University of Melbourne in Australia. This, combined with the fact that increased blood flow and endorphins make you physiologically aroused, consumes more brain power.
In that state, your mind defaults to making automatic responses, causing you to share information that you typically try to conceal, like insecurities or fears.
The bottom line:
To build a stronger bond with your workout partner, opt for a self-led run or strength session rather than a group fitness class to allow for conversation.