it's undoubtedly psychological and not spiritual, it's just a plant that stimulates certain receptors in your body. but everybody responds to weed differently, some more than other. some people can become depressed, or in your case have "feelings of darkness", which are still feelings. the other option is that there's a darker side of your personality that's coming out when you smoke.

but to each his own, I say just don't smoke it, it's obviously not for you. for every so many people who have a great and typical time on weed, there's always the odd person who just gets a really negative experience from it.