Satan is never content with trapping a person in a single sin. There is always a possibility of escaping, so he drags a person deeper into sin to make escape look impossible.
You can see this in your own life. You've told me a number of times that alcohol has never been a problem for you because you just don't like the taste, yet as you became firmly addicted to marijuana, you turned to drinking frequently. There is a simple reason for this: because marijuana is illegal and costly you don't always have access to it, yet the withdrawal symptoms begin within eight hours of your last use. Thus people turn to "legal" drugs -- cigarettes and alcohol -- to help handle the withdrawal symptoms until they can get their next hit of marijuana. But these drugs are addicting in their own right, so repeated use makes you addicted to them as well.
Then there is the problem of how marijuana changes your ability to make judgments.
"The short term effects of marijuana use include: memory loss, distorted perception, trouble with thinking and problem solving, loss of motor skills, decrease in muscle strength, increased heart rate, and anxiety." [National Institue of Drug Abuse, Journal of the American Medical Association, Journal of Clinical Phamacology, International Journal of Clinical Pharmacology and Therapeutics, Pharmacology Review.]
A marijuana user tends to become compliant when stoned. His inhibitions against doing things he knows is wrong or harmful is weakened. Thus, when offered harder drugs, a marijuana user doesn't think clearly and is willing to take greater risks without considering the consequences. There is also the fact that a sinner, once caught up in sin, is easily persuaded that one more sin isn't so bad.
"According to CASA (National Center on Addiction and Substance Abuse), a 12-17 year old who drank alcohol and smoked cigarettes even once in the past month is 30 times more likely to use marijuana than one who did not; a child the same age who has used all three drugs in the past months is 17 times more likely to use another harder drug (such as cocaine or heroin). These statistics apply to children who do not already display problem behaviors." [Lura Seavey, Gateway Drugs].
"But evil men and impostors will grow worse and worse, deceiving and being deceived" (II Timothy 3:13). Sin, by its very nature is progressive. At the same time, those caught in its trap deny they are trapped and that they are sinking into a worse state. Don't let Satan fool you, my friend.