Why should I split password reset and username process apart?

I was looking at a similar question, here , for some insight but it does not answer my question. I am developing a page that users get to by clicking a simple link titled "can't log in?". I was told by a senior that I should split my username and password recovery into two screens. As a user who can't log in, however, I would think having both resets on one page would be a much better experience. I can have my username emailed to me while staying on the same screen, then immediately type it into the password reset to get a link to reset my password. His reasoning is that "other sites do it this way" and "it is easier for users to know what they need to do."

In this project, I have no choice other than to follow this suggestion no matter how much I detest it. In the spirit of understanding why, however, which side do you agree with and why?

Edit: I think it is helpful for me to mention that over the course of a couple years we could reasonably expect these emails to change for our site. That is why username has been chosen to use instead of email for login.