Why won't my dog eat his dog food?

When I got my dog he ate his dry dog food with some wet food mixed in. Then he stopped eating it and we switched Beneful Prepared Meals, and he ate that for a lil bit. Then he stopped eating that. So we tried other foods but he didn't want those either. We started giving him some of our food (bad idea, I know, but I didn't want him malnourished :/) and now he won't eat it unless we hand feed it to him. How do I get him to eat his dry dog food and not our human food?