a game every one love to play...

I suppose CS has a great life span due to its simple yet hard to master nature. Few games can be as successful as CS, although the game itself is very simple like all other shoters, but is very hard to master it, as there are much more inside the game than its controls. It doesn't have any flashy special effects (except the flash bang), is not bloody, and is just pure competition. It has few things to distract the players, and even playing on the same map over and over can have different experiences, as human behaviour's randomness is also part of the fun. Like all HL1/2 related games, the core game mechanics are really amazing, you can create your own maps, study other's battles, and just have fun, anyone likes HL series will sure enjoy CS/CS source.

i love to play that game. i am obsessed with it. i have to install it in my new computer first. counter strike and half life are awesome games!

You mean Counter-Strike? I dunno, No specific reason, It just so addictive, ahaha ^_^ It's super fun ^_^