There are many factors that play a role when it comes to a person's individual risk for becoming addicted to a substance or habit. Here's what the National Institute on  Drug Abuse for Teens has to say:

Great question, and a hard one. We don’t fully understand yet why this is so. We know that genes play a part, because an inclination for addiction can run in families, and because different strains of mice, rats, and other animals differ in how readily they develop addiction-like behaviors after they’re exposed to drugs. We also know that a person’s environment plays a part in addiction. For example, what are the factors that encourage someone who has tried a drug to keep on taking it to the point where they can’t stop? Many scientists are trying to untangle the answers so that we can find better ways to prevent and treat addiction. See these videos on how anyone can become addicted, and why drugs are so hard to quit.

Update: Read more on what increases the risk of having a drug problem.

Source: Eight Questions from Teens About Drugs & Alcohol

It can be hard to figure out what to do when you suspect that someone close to you is abusing substances. Here are some tips on what to say from the National Institute on Drug Abuse for Teens:

• Just telling your friend that you’re concerned can be a big help. Your friend may not want to talk about it, and the effects of drugs on the brain may keep him from “hearing” you or acting on your advice.
• Assure your friend you are there for her and that she is not alone. People with drug problems often have gotten in with the wrong crowd—and they don’t want to turn away from these so-called friends for fear of being alone.
• Suggest that he speak to a trusted adult who will keep it confidential. Maybe there’s a family friend who could help.
• Turn to a professional for immediate help if the problem looks to be too big for you to handle alone, or if you’re worried your friend may have suicidal thoughts that she could act on.
• Use SAMHSA’s Substance Abuse Treatment Facility Locator or call 1–800–662–HELP to tap into a support network where you can find immediate and confidential help 24/7. They'can also direct you to local treatment options.

When the people we care about and have lots in common with make bad choices, it can be frustrating, confusing, and a little depressing. Still, we should be there for our friends—and also try to be a good role models for them by making smart choices ourselves.

Source: National Institute on Drug Abuse for Teens

Good question. Here's what our friends at NIDA for Teens have to say on the subject:

The “gateway drug” concept—where using one drug leads a person to use other drugs— generates a lot of controversy. Researchers haven’t found a definite answer yet, but as of today the research does suggest that, while most people who smoke marijuana do not go on to use other drugs, most teens who do use other illegal drugs try marijuana first. For example, the risk of using cocaine is much greater for those who have tried marijuana than for those who have never tried it. However, this risk is also greater for people who have used alcohol and tobacco. Animal studies suggest that because the teen brain is still developing, using marijuana, alcohol, or tobacco in your teen years (or earlier) may alter your brain’s reward system (see the answer to #3 above), and that may put teens at higher risk of using other drugs. In addition, using marijuana puts children and teens in contact with people who use and sell other drugs, increasing the risk of additional drug use.

With mental health problems so prevalent among today's youth, it can make you wonder whether young people are more likely to develop them. Here's what the UK's Mental Health Foundation says on the subject:

There are certain risk factors that make some children and young people more likely to experience problems than other children, but they don’t necessarily mean difficulties are bound to come up or are even probable.

Some of these factors include:

• having a long-term physical illness
• having a parent who has had mental health problems, problems with alcohol or has been in trouble with the law
• experiencing the death of someone close to them
• having parents who separate or divorce
• having been severely bullied or physically or sexually abused
• living in poverty or being homeless
• experiencing discrimination, perhaps because of their race, sexuality or religion
• acting as a carer for a relative, taking on adult responsibilities
• having long-standing educational difficulties.

Source: Mental Health in Children and Young People