Couples who smoke together, stay together, according to a 2014 study published in the journal 'Psychology of Addictive Behaviors.' The study followed over 600 couples for a span of 9 years and measured their behaviors over that time.
Researchers found that couples who used cannabis were less likely to have violent relationships compared to non-users. This is also the case if only one partner uses marijuana but not the other. The study suggests that those positive results largely come from the fact that marijuana provided couples with a shared interest that also reduced anxiety.
The study also noted that the illegality of cannabis in the US at the time of the study may have impacted the results. The couples had to be confident enough with each other to participate in illicit actives together, displaying a certain level of trust. Imagine a Bonnie and Clyde relationship, but with joints instead of tommy guns.
Of course there are also the numerous ways in which cannabis use affects sex, which the study cited as another reason why cannabis users tend to have stronger relationships.