What are the negative effects to the first author when adding coauthors?
I think for some people, the principle is important: they didn't feel that the person whose name has been added contributed enough, and humans hate the feeling of giving up something unfairly.

That said, there is a very real cost in terms of credit.  This will not be consistent across different people looking at your record, but every new co-author decreases the chance that the important part of the work will be attributed to you.  There was actually a recent study about this which found that women pay an especially large price on this score: when they do joint work with men, the evidence suggests the men tend to get the credit (the actual observation of the study is that their chance of getting tenure do not improve).  You'll note though, even for men a solo paper improves chances of tenure by more than a jointly authored paper with anyone.