Age, strain,genetics. Marijuana is hazardous for young people- regular users will have lower iq til about age 17. Some people become very anxious from it- this is related to your genetics/constitution but can also be influenced by the strain as diff. Strains have varying ratios of thc, cbd etc, thus some are more calming ; agitating. Like all medicines, what helps some can harm others ; it is not for everybody!
Innate vulnerability. Marijuana can be especially dangerous for people who have a known or previously unknown biological vulnerability for a psychotic illness (similar to schizophrenia). Of course, it can worsen pre-existing respiratory diseases (copd, asthma) and cause people to gain weight. Hope you are thinking of taking this up. If ur already using consider quitting. Speak to ur doc about it asap. Good luck!
Marijuana. I am not sure i understood the question correctly. Marijuana in and of itself can cause adverse effects such as increase in anger, suspiciousness and even psychosis. Long term effects are memory impairment, inhaled marijuana may lead to lung cancer. You don't have to have any changes in your wellbeing for these effects to manifest.