Some reviewers are actually annoyed by the number of requests from journals. Peer review is important, but it is an additional burden for researchers. Perhaps this work should be better acknowledged. See these related discussions:
I know that when i am a reviewer that i am unable to fulfill every request so i now can only accept requests from the better journals because i receive so many requests. I do not best to accept as many as possible. Currently i estimate it is about 85% still.