At the beginning it was Quake and Quake 2 then Half Life - but they all were kinda fast paced and not so realistic, surviving a rocket collision always intrigued me.
Then Counter Strike appeared, with bots and maps and everybody love it, de_dust2, de_aztec, there was no lanparty without CS1.6. I remember that at a certain point we learned how to buy weapons just by using numbers and other shortcuts, it was getting more and more serious, jumping with the knife was crucial - if you wanted to avoid being called bot, noob, etc.
AK 47 aiming for the chest, you know the next bullet would be a headshot, these flash bangs that worked always against you, jumping on top of your opponent so you got it cover 180 degrees, man that game was tactical.
I played CS1.6 last month, it looked so ancient and simple, these 700 hours of Battlefield 4 changed me, I didn't remember I've played that much CS, even if I did there were no stats, no servers, no fancy knife skin, just get your weapon and GO GO GO.
And remember, if you find a toilet in CS 1.6, just don't shoot the ...