If your using a strain that has high THC and not much in CBD. CBD being the compound to safe-guard, governor the effects of THC you end up in a trip that can spiral out of control causing negative effects.
If you add a CBD strain to the mix. Which are high in CBD and contain trace amounts of THC to your normal method usage, these tend to level out the percentages.
My take is this: If your not mentally fit, high THC can cause you to go off the edge compared to strains with lower amounts. The same can be said for Wine and the alcohol volume.
And I talk from personal experience from being in a psychosis episode caused by cannabis.
-- Edit: I have my own downvote brigade. Yay - Just seems odd that every post I post on HN gets down-voted.