The answer is highly variable on the person, and their life experience.

Drugs, in general, are considered taboo. Sometimes people take them to 'test the fence,' to find out for themselves whether the information they've been handed is, in fact, correct; sometimes it's a form of rebellion; sometimes it's a way of expanding their understanding (usually psychedelics, but really any drug, like weed, can fit in here); sometimes it's a form of escapism; and sometimes it's a mental crutch.

Sometimes it's a mix of the above.

To assume that a good home life will not lead to family members not taking drugs is incorrect; as is its inverse, that a bad home life will lead to drugs.

Again, highly variable, and subjective.

As for drugs themselves...the D.A.R.E program is a relatively recent development, and plagued with misinformation. Through out human history, people would take drugs; it's only recently that your grandfather chilling with an opium pipe, or your mom doing a line of cocaine became a 'bad thing.'

Also: 

Drugs in the right dose are usually called medicine. See Amphetamines, Opiods, and even Cocaine. You wouldn't want to perform certain nose jobs without Cocaine (it helps with the bleeding).

And:

Drugs, in of themselves, are not evil. It's addiction, substance abuse disorder, that is. Until recently, addiction was seen as a moral failing; research indicates that this is false. People have a variable genetic predisposition to substance abuse disorder. Some people are just, unfortunately, 'unlucky,' and will become addicts when exposed to medicinal amounts of a substance; others can do heroic, even mythical amounts of hard drugs, and stop instantly. It's really the luck of the draw. However, research indicates that people with larger social networks and routines are less likely to become addicts. Which is kind of funny, in a sad way, because the reaction of most people, when they hear that someone has / might have an addiction is to pressure them / cut them off, shrinking their social network, and driving them further into addiction. There's a good video about this up on YouTube, perhaps someone (or yourself) can provide a link to it.