They don't think ahead because they don't have the empathy and the foresight to anticipate any of the negative effects happening to them. They think about the here and the now, and not the there and the later. For example, kids start smoking because of peer pressure and/or how it makes them feel. The don't, however, think about how lung cancer is the leading preventable cause of death in America (I believe it's number 1, but if it's not, it's certainly high up there. I think second is obesity, or something like that) and if they do, they don't think it'll actually really happen to them.
Now, drugs like marijuana which aren't really that harmful to your body if you don't abuse them are used primarily because it's fun. There's no other reason, really.
And then they get addicted and they can't stop, so they just keep doing it.
In addition to physical and/or psychological addiction, to a lot of people, their lives just seem dull and drab without drugs. Why? Because drugs affect the limbic system of your brain and cause you to produce less dopamine than you normally would. So, the only way to even get your dopamine levels up to normal is by using drugs until, of course, you break the addiction and get your brain back to what it used to be.
People have always used drugs and probably always will. Not all of these people are stupid. There are some incredibly smart and intelligent people that smoke cigarettes and marijuana, but they keep on smoking because it's an addiction.
