Winston says this because he cannot think of any other reason why he should be dissatisfied with the physical conditions of his life.
When he thinks this, he is sitting in the cafeteria at work and he hates everything about it. The food is bad, the cafeteria is dingy, everything there and in other places is just poor quality. But then he thinks that he cannot remember that things were ever any different. If you have no memory of things being better, how can you be dissatisfied? Winston decides that the only reason for him to dislike the way things are now is if he has this ancestral memory that tells him that things used to be better (in some previous generation).