Not smoking weed is a much better life choice than smoking it.
Weed takes you away from reality. It's also illegal, so a nice risk of getting arrested. It makes you less productive, makes you lazier and it also costs a lot of money.
If your cousin wants to smoke weed, then let him, very few people who smoke weed go on to be extremely successful as you only become successful from hard work and weed is about the least productive thing you can do with your time.
If your life sucks that much that you need to rely on smoking a drug to make it better, then maybe you need to find a way of making your life better.
I've nothing against people who smoke weed, it's their choice, but don't smoke it because your cousin does.
Also of course medical issues such as paranoia and schizophrenia.