The academic review process is usually supposed to be blind, in that authors are not told the identity of the reviewers. But frequently it is possible to guess who has reviewed your work, or the reviewers themselves may let you know informally.
Say my manuscript is rejected from journal A after one very positive and one more critical review, and I think I know who the reviewers were. After some revisions I submit to journal B, who ask me for a suggested list of reviewers. Is it ethical to list the reviewer whom I suspect gave the positive review the first time, but replace the one I suspect has a negative opinion of my work with another reviewer? What if the positive reviewer is someone whom I did not originally suggest to journal A - is it ethical to suggest him/her to journal B in the light of their previous review?