You can chant about anything!
We have experienced something personally that has led us to believe that the ultimate Law of Life and karma is, cause and effect. There is individual karma and there is collective karma. Why do all of the people from varying backgrounds sometimes have collective fates (9/11 for example)?
We believe that the environment reflects the collective karma of those who inhabit it. We have come to accept a view, based on personal experience, that there is a means for individuals to change their Karma, and bring out their Buddha Nature. That is why we chant.
The belief is that the more people that chant, the more the environment of all living things is influenced as an effect of that cause. We are all devoted to achieve happiness in our daily lives and to hopefully express to society through our efforts, a reason to be curious about this NAM MYOHO RENGE KYO, and try it for themselves.
We believe at a point in time in the future, through the efforts of really nothing more than our daily practice, we can influence the numbers of people it will require to "Pacify the land, through the propagation of the law." Kosen Rufu. I.E.: If enough people chant, the ozone layer will heal, the droughts and floods will stop, the earth will become benign to it's inhabitants and people throughout the world will respect the sanctity of life, based on respect for the Buddha Nature in all things.
I never thought I would be discussing it in a reefer forum, but there you have it.