Well, if you'll allow everybody to review everybody PRs then eventually that's what you'll get.

Similarly - junior might look at your PR and see that it's ok. Senior might look at it, see that you've forgot about a thing that junior didn't know about, you omitted to test a certain condition, etc.

Again, juniors are more than welcome to look at PRs and be a part of discussion, but if you're merging, you should be wary. I'd personally be against allowing a merge if you get only a junior approval.