Why do teenagers think smoking weed and marijuana is cool?

[Slient_Love]

I'm 17. I have never done any drug before. My cousin on the other hand is a bit more rebellious than I am. She has her own apartment and has a kid who's 3. She drinks but she doesn't know that I know about it because I kind of stalk her on twitter. lol. She said she has been sober for about a month.
I want to know why almost all the cool/popular people I know or met have either said they've gotten high on weekends/smoke and drink and said it was fun and cool and won't affect you.
But what I think is that it will affect you later on in your late adult life.
They usually say they smoke because they want release from problems or a stupid answer like because it's fun. I always say when I get mad, sad or feel like crap I write and play my keyboard(Piano) or draw. Writing is my way of escaping instead of thinking nothing would go wrong with me for smoking or drinking. Because for me I think smoking and drinking would affect you later on in your adult life you just don't notice it now.
What's your take on teenagers smoking or drinking?
I think it's awful.
Could it affect them later on in their adult life?
From my personal experience. From last Summer hearing how my cousin's attitude changed towards her four year old son was for the worse as in the yelling part of it. She's just lazy with him. I want to take care of him for some time during the Summer because I know how she treated him when she didn't drink or do anything stupid.
The drug that I'm focusing on is weed and marijuana. I find it ridiculous that other teens think it's okay to smoke