I don’t think Oliver is being ungrateful at all. Oliver is being completely honest because he felt that he could love someone, a human, other than just a dog or another cat. When you think about it, a dog living on the streets has a rougher life and has issues in trusting people because of their lack of socializing. From the way Dodger is acting, I do believe that he is among a few of the dogs who had the experience of being adopted in a family and his family either treated him poorly or they left him and never came back. This gave him a result of wanting a “family” of dogs and cats living on the streets. Being exposed to one family that get bad results can easily effect a dog, or even a person, of the family. This comes into Dodger overreacting towards Oliver and his strong belief of humans being nothing but bad news.
"When I'm 80 years old and sitting in my chair, I'll be reading Harry Potter. And my family will say to me, 'After all this time?' And I will say, 'Always.'" - Alan Rickman