Why do people choose to use slang when they can use other words?

First of all, I have to admit that I have found myself in this line, and indeed, why do we all use slang beside all those appropriate words? For example, why would we rather say “bad egg”, instead of “bad person”, or why do we say   “cool” which is absolutely the best example?  I’ve got an explanation.

Slang is a modern language, very changeable, which is, in the most cases, made and used by the young generations. During the past years every generation had its own slang. That was their language, their own mark. People use it in order not to be different from their society. That characteristic, being a part of  the environment, is in human nature and I’m afraid it is quite hard to resist it. Slang easily becomes a part of your life and you just become a person who cannot even remember when the changing  of vocabulary started.

But, years will pass and they will bring a new language shape. Maybe some day “cool” will be completely expelled from modern language, or…it will be replaced with some new popular word with two “o”s. You never know…