What are the negative effects to the first author when adding coauthors? There are many questions on this site which involve authorship disputes and ethicality. 

In fact, there are often people who are greatly distressed when their contributions to a paper are "lesser" than they expected, for whatever reason. This could range from having a co-first author added, or having other secondary authors added to the authorship list. Another famous example woudl be the Alpher-Bethe-Gamow paper regarding Big Bang nucleogenesis, where the first author Alpher greatly resented the addition of Bethe to the authorship list. 

However, I do not understand why adding co-authors would be a bad idea. Surely, the fact that you were helped by another person means that they should be co-authors on the paper, as long as they made significant intellectual contributions. 

None of the fields mentioned in this question regarding authorship norms had any explanation of the potential detrimental effects additional authors have to the first author. 

What, then, are the possible negative effects that the first author can suffer if other authors are added to the paper?

Note: I do not intend to ask regarding issues where the original author is removed from consideration, only those where the first author is kept, but other coauthors are added to the authorship list.
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.