No idea,None of my friends ,male colleagues ,brother and my dad are like that.

Probably because they like you? Did they ever blow a kiss your way?

It depends on where you go.

In Guilin, and in Beihai city, it was rather common to see this, along with pant legs rolled up to the knees, etc. especially in the summer time. In Beijing, I see it, but it is not quite as common and the locals give looks of disapproval

As to why, it is to keep cool in the warm weather.

In SZ this is class related.  Office workers will avoid the sun at all costs simply to prove to the public they are better because they don't have to work in the sun.

The only guys that do this are migrant/factory workers that don't have that complex.

An the fact that they are all skinny and not fat doesn't hurt the cause.

Because real, macho Chinese men go to the massage palour to have the hot cup treatment thing. They have to show off the red or brown circles all over their backs to show how brave they are!

It's sad the ladies don't also do this.... all those fine young girl in Dongguan walking back to their dormitory after a long day grinding metal... then again, if aunties did that too.. I'd be blind~ 

I'd say that it's a summer thing -- I see it around here all summer long.  Most of the time the guys are rather fit and well-toned, sometimes not but then again I live in a beach town and in the summer a beach mentality pervades the entire city.