How can people get so many kills in Call of Duty games?

I always seem to get mad or 'jealous' of the other players getting so many kills. For instance 35-2... I mean I get so mad that I message them and tell them they have no life. I'm so immature but I can't help it. I HAVE NO LIFE and I admit it and I play it all the damn time. I get around 20 kills a game but I get like 10 deaths and such. Always half. How are these other players so good? Do they really sit on their asses 24/7? I play probably around 7 hours a day and that's still sad but whatever. My friends are always busy working and I am too lazy anyway