My mom tried to teach us ways to stay healthy, but i got arteritis. why is that?

You did not cause it: Arteritis is inflammation of the blood vessels. It depends on which ones were invovled. Viral infections can cause certain types of arteritis. You can get viral infections even when keeping as healthy as you can. Other types of arteritis can be associated with medical illnesses that can occur no mater how healthy you try to stay. It happened but not due to anything you did or did not do.