What are the pros and cons of writing papers alone?
The advantage of writing with someone else, someone more experienced in particular, is that you may end up publishing quicker since you're not doing all of the work. It can also allow you to write papers that you may not be able to write on your own assuming the co-author(s) bring(s) something to the table that you can't offer. For graduate students, it is a good idea to write their first paper with someone more seasoned so that they also get advice on the logistics of publishing a paper (how to pick the right venue, how to write the cover letter when you submit, how to respond to reviewers - some of these an advisor can help with regardless of co-authorship, but a co-author will be more invested). In some fields, it may be frowned upon that you have no co-authored papers as it may signal a bad relationship with an advisor or that you are generally hard to work with. This part is completely field-dependent. The downside of having too many co-authored papers is that people may wonder whether you can work independently.