Answer:
It can be genetic, but it can also be from chemicals you may have been around or poor diet, like some type of imbalance in your body. I have Rheumatoid Arthritis and was diagnosed six years ago. I'm still doing my research, but feel I have learned a lot. Although, I am still trying to figure out the main cause of it myself.
In the main The cause of arthritis is to date not understood and there is no cure for it. there are however a hundred different form of the disease with various treatments.