Writing your own recommendation letters can seem like a blessing. Suddenly, you have the power to control an aspect of the application process that was previously beyond you. So, your downside risk in these letters is mitigated and your upside is infinite, right? Well, it does not quite work that way.
Admissions committees are not seeking blustery rave reviews, but are seeking recommendations that are detailed and personal, intimate and sincere. Can you really write about yourself with a dispassionate sincerity? And, even if you are a master of dispassionate sincerity, are you able to capture the subtleties that enable you to standout? For example, let's say that among the many important things that you do, you also do something thoughtful, which you do not even perceive to be significant - you take new team members to lunch. While you regard closing the big deal as significant, others may appreciate and admire this small and impactful act, which forges team unity. Unfortunately, you may lack the objectivity necessary to ensure that this nice detail makes it into your letter.
This is but one simple example, but our point is that you probably will not know what is missing from your letter if you write it yourself. So, when you approach your supervisor for a recommendation, go in ready to push back a bit if they ask you to write your own letter. Some may be busy or lazy and others may think that they are doing you a favor by giving you control. Be prepared to impress upon your recommender that you can't help yourself and that he/she can. After all, that is why you are approaching him/her in the first place!