THC is the main psychotropic cannabinoid in cannabis simply meaning that it is the main one that alters your mental state. That is not to say that other cannabinoids do not do the same, THC just does it better.
The reason THC causes this very natural effect is due to there being a concentration of CB1 and CB2 receptors within the nervous system that interact directly with THC molecules. These receptors are accustomed to this type of interaction because it is a process that already takes place, or at least ought to, in your body regularly using a molecule very similar to THC called Anandamide. Anandamide plays a role in eating and sleeping habits, pain relief and happy feelings, the kind of feelings that put a smile on your face and allow stress to melt away.
THC can be used in varying doses to supplement the body for the same reasons and ‘getting high’ simply describes the effects felt after over imbibing this particular cannabinoid. Someone who is new to supplementing with THC ought to consider choosing products that are relatively low in their content of this cannabinoid (less than 10% by dry weight, 5mg’s or less per ingested dose) working their way up in order to find a dose that fits into their life. Consider balancing with CBD and CBG for a well rounded effect, and if you find yourself with an undesired high, try counteracting with CBD.