Many people will attest to the potential for cannabis to trigger paranoia and anxiety, while many others will attest to the benefits they receive in reducing anxiety and battling similar symptoms.

The contrasting experiences different people report, and the contrasting experiences the same people report at different times, is evidence of the complicated relationship between cannabis and the brain, especially when dealing with emotional reactions.
Cannabinoid receptors are abundant in the brain, including many in the amygdala, which is associated with emotional responses, like fear, stress, anxiety and paranoia.
Just as many recreational users will attest to the potential for cannabis to trigger or exasperate paranoia and anxiety, many PTSD patients, among others, can attest to the potential for cannabis to relieve these very symptoms.

The Endo Cannabinoid System is heavily involved in maintaining mental health, which is why cannabis can be very useful in treating mental health problems in some situations, but is also part of the reason why it can induce negative responses in people.
If your body is not producing sufficient cannabinoids to keep your brain healthy then cannabis may prove a valuable medicine, especially for relieving symptoms and even as part of a treatment, however if the problem is not as simple as a cannabinoid deficiency or involves an imbalance that cannot be rectified by cannabinoids then cannabis may be ineffective or even detrimental, at least regarding its immediate effects.
Unlike other medicines, there are no real dangers of tangible harm from exposing the brain to large doses of cannabinoids, but it may not be what is actually required and as a result may provoke undesirable reactions, with negative emotional responses that include paranoia.

One thing that has been heavily suggested by the anecdotal reports of cannabis causing paranoia is that personal circumstances appear to be equally relevant, if not more so, than the cannabis.
Different strains may bring about different responses, but also different people and the same people but in different circumstances all give different results.
It is not always easy to pin down the deciding factors but obvious variables are sometimes evidenced in different experiences people report.
For example, a common report from many people who smoke cannabis have reported paranoia only when in a situation they were already uncomfortable or where a looming threat exists, like smoking in an area where police may potentially catch and arrest you. Whereas smoking in their own home or with friends they trust, or in a place where criminalisation is not an issue (like a coffee shop in Amsterdam) rarely involves such symptoms.
These experiences lend a lot of weight to the idea that, rather than outright causing feelings like paranoia or anxiety,
cannabis simply heightens these feelings and/or brings them to the surface, often to the point that it can feel like an outright manifestation.

Another factor that appears significant, according to many reports, is the cannabinoid (and even the terpene) profile of a specific strain. A common report is that strains with large CBD content (not necessarily higher than THC but close to equal at least) is far less likely to induce paranoia and/or anxiety than strains that are very high THC with very little, or no significant quantities of, CBD.
Just as the Endo Cannabinoid System (ECS) produces and utilises different Endo Cannabinoids for a variety of purposes the different cannabinoids in cannabis provide different therapeutic effects, some of which will be profoundly beneficial, some of which will be benign and some of which can even be detrimental (though not actively harmful).

The simple truth is we do not know with a 100% established certainty why cannabis triggers or exasperates paranoia but we can infer from what we do know that the more compelling factors to such symptoms are the underlying issues, the predisposition of the person and their environment, circumstances and mental/emotional state.
Though it cannot be ignored that many people have reported feeling paranoia when using cannabis, while insisting there is no other potential reason for such effects, it also cannot be ignored that many people have reported very different experiences when their circumstances change. Add to the that the unquestionable benefit cannabis can provide to many sufferers of many different types of mental problems, including (but not limited to) schizophrenia, bi polar disorder, PTSD, severe anxiety, acute depression and paranoia.

Just as the interaction between cannabinoids and the cannabinoid receptors in our brain can be used to prevent seizures in sufferers of epilepsy, cannabinoids can also be useful for addressing imbalances that contribute to or cause other problems in the brain.
The same network that can relieve severe anxiety in many people also increase it in others, or even in the same person in different circumstances. And like with epilepsy some cannabinoids can be more useful than others and as such the cannabinoid profile has great importance.
When it comes to paranoia in particular, there are so many factors that play a role and so many varying and contrasting experiences that it is impossible to say with 100% certainty why some people experience paranoia every time they use cannabis, while others experience it only from certain strains, or in different circumstances, and many more people simply never experience paranoid feelings from any forms of cannabis during any situations.