Finding Out That A Coworker Is Shady Behind Your Back?

I've had a coworker that has been in the same workplace as me for four years, we've gotten closer over time. Just for me to find out, that when people try to praise me in any form he would always try to downplay my performance etc. in front of other people. For example, someone could be trying to talk about me in a positive light or expressing that they admire my hard work and they will "jokingly" put me down instead of lift me up.

I'm the opposite in that I praise and defend them in front of people. Why is that?

Finding Out That A Coworker Is Shady Behind Your Back?
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