Lots of things happen when a state legalizes marijuana, and some things don't happen.
Here's some easy to read charts that will help.

Arrest for marijuana drop through the floor, but it isn't even.

A 2016 report from the Colorado Department of Public Safety found, “The decrease in the number of marijuana arrests by race is the greatest for White arrestees (‐51%) compared to Hispanics (‐33%) and African‐Americans (‐25%). The marijuana arrest rate for Whites and Hispanics is comparable, but the marijuana arrest rate for African‐Americans is almost three times that of Whites (348/100,000 for Blacks and 123/100,000 for Whites).”
This is still progress for marijuana legalization advocates, since fewer people of all races are arrested for cannabis in the end.
But the findings also show the persistence of racial disparities in the criminal justice system.

International organized crime leaves the market due to reduced profitability.

"Cartels know their ability to compete in the U.S. marijuana market is diminishing," said John M. Walsh, director for drug policy at the think tank Washington Office on Latin America. "U.S. consumers have better options."
Marijuana's profitability has been greatly reduced, with farmers receiving much less than they used to for cultivating the plant.

Teens don't start smoking it. Maybe because it's no longer cool.

This headline is alarming: Exclusive: Traffic fatalities linked to marijuana are up sharply in Colorado. Is legalization to blame?
But does it actually mean anything?
Consider two other headlines from the same time.
Colorado's Marijuana DUIs Are Down 33%
Marijuana legalization has not increased traffic fatalities

In fact, marijuana legalization has not increased overall traffic fatality rates nor the total number of non-fatal crashes, according to two separate studies conducted by Columbia University and the University of Texas-Austin.

Additionally, Colorado State Patrol reports a decrease in the number of driving impaired accidents since marijuana sales became legal.

Despite this academic research and on-the-ground expertise from state police, it was a study from a group supported by auto insurers last week that captured the most – and loudest – headlines. “Car crashes leap in states with legalized marijuana,” was just one of many headlines characterizing The Highway Loss Data Institute’s (HLDI) finding that three states where marijuana is legal – Colorado, Washington and Oregon – had an increase in traffic accidents.

However, the group lacked any data showing that the increase in collisions was caused by drivers under the influence of marijuana. Although distracted driving such as texting, road construction and more miles driven have been noted by insurance companies as possible factors for increased collision rates, the HLDI asserts that because there was an increase of insurance claims in states with legal marijuana, the two must be linked.

Lastly, there is the issue of taxes and jobs.

VS Strategies, a pro-legalization research company in Denver, says the state has pulled in $506 million since retail sales began in January 2014. That includes taxes and fees from medical marijuana, which was legalized years earlier, but the vast majority of the revenue came from recreational.
Revenue from taxes and fees has increased each year, from $76 million in 2014 to $200 million last year, and the state is on track to beat that this year, according to VS Strategies, which used state revenue data in its report Wednesday.
Colorado has spent most of the money on schools, the research company says. Smaller chunks have gone to drug prevention and treatment programs and to regulating the marijuana industry

If Trump/Sessions can be sent home the affects could be huge.

Marijuana legalization could create $132 billion in federal tax revenue and inject over a million jobs into the US labor market by 2025 if it becomes legal nationwide, a new study says.

The study, from cannabis industry analytics firm New Frontier Data, seeks to estimate the total economic impact of the nascent industry. Cannabis is legal in eight states, including California, which legalized recreational sales on January 1. Vermont is likely to join that list once Gov. Phil Scott signs a bill legalizing the possession of up to one ounce of marijuana into law.

The study assumes the tax revenue, which will add $131.8 billion cumulatively to the US Treasury by 2025, will come from a 15% retail tax, payroll tax deductions, and a 35% business tax. Sales tax alone on cannabis would add $51.7 billion to US coffers between 2017 and 2025.