Lou Diamond Phillips is sharing Tim Robinson's secret sauce ... he says the reason "The Chair Company" is a hit is because it's so relatable.

We got Tim's costar on the HBO series at LAX and our photog asked him why folks are going nuts over Tim's latest offering.

Lou says pretty much everyone can relate to being on hold for hours on end trying to get some customer service ... and he says that's the reason 'Chair Company' is now in the national zeitgeist.

Even a huge celeb like Lou isn't immune to these same sorts of issues ... and you won't believe how long he's been stuck on hold waiting to hear back from a person who may be in India, the Philippines or somewhere else.

It's a fun conversation and Lou tells us when Season 2 is going to ramp up production ... so prepare for more of Tim's genius -- and relatable -- comedy gold.

Stay tuned ...