You will be shown an excerpt from a computer science scientific research paper, followed by a question about the paper. Please answer the question. Do not include any text in your response other than the answer.

Context: Nevertheless, the negative sampling approach has critical limitations in the following aspects.First, the underlying assumption about negative interactions becomes less valid as user-item interactions get sparser.This is because as fewer positive interactions are observed, the number of ”positive but unobserved” interactions increases, which consequently makes it even harder to sample correct negative ones.Such uncertainty of supervision eventually degrades the performance for top-K recommendation.Second, the convergence speed and the final performance depend on the specific choice of distributions for negative sampling.For example, sampling negative pairs from a non-uniform distribution (Rendle andFreudenthaler, 2014; Dinget al., 2019) (e.g., the multinomial distribution which models the probability of each interaction being actually negative) can improve the final performance, but inevitably incurs high computational costs, especially when a lot of users and items should be considered.

Question: Why does assumning unobserved user-item pairs negative leads to limited performance for generative methods?
Assuming unobserved user-item pairs negative leads to limited performance since there are some cases of positive but unobserved, and the number of this case is increased.