Why Do We Love Valve?

Here's a thing, many games have been released by numerous companies over time. Some instant classics and unbelievably enjoyable others not so good. Many games have spanned different companies and genres and sometimes just been produced to death. Whilst some have been one hit wonders and never touched again. Why are Valve games the ones we keep coming back to time and time again to play?

As of this date Counter-Strike Source is the 4th "most played" game on X-Fire, only being beaten by the Call Of Duty franchise and World of Warcraft. All of Valve's major online games appear in the top twenty list. Surely now the Counter-strike horse has been beaten to death, the original game having been officially released ten years ago.
What is it about these games?