Derek Hough ’s celebrating 'Dancing with the Stars’ big 2-0 -- which just so happens to line up with TMZ’s own 20th birthday -- and he’s spilling the secret sauce that’s kept both of us swinging strong all these years!

The pro dancer helped us shake off the Monday blues ... chatting at LAX about how both the dance competition and our celeb news site have kept fans hooked for years -- thanks to one thing we share in common: authenticity. Gallery Launch Gallery Getty

Derek says people love a no-holds-barred look at others in totally different settings -- and that’s exactly why both 'DWTS' and TMZ have thrived ... keeping it real and raw, no filter needed.

Catch the full clip ... 'cause Derek spills even more of 'DWTS’ goodness, including his all-time favorite moment on the show,

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