As a reformed people pleaser, I certainly understand why being selfish when it counts is good for you in a variety of aspects of your life. With regard to mind and mood, when you are selfish you can be honest and not care so much about others to your own detriment. Some of those will be those who don't care much about you. I used to spend a lot of time worrying about others...some of whom never gave a darn about me!
Being selfish can boil down to more time for you to focus on the things and people who are important to you. With this focus you are more likely to accomplish greater deeds in number and importance than a people pleaser ever would. Being selfish can prevent you from wasting time or opportunities because you want to "be nice" or be a "team player" and allows you the ability to ask for or demand what you deserve rather than waiting for others to greenlight it for you (good luck with that). Being selfish in the right situations will put you further ahead than being a people pleaser ever will.
Some selfish people may be respected simply because they do not exhibit the people pleaser mindset and they demand what they want, need, or have earned. Their self esteem and confidence is not dependent on keeping the peace and avoiding confrontation. For example, I recently saw a news clip about the fact that most adults who would rather go without an impossible number of things, literally including going without money, than to ask for or negotiate a pay raise.
In this scenario, those who are deemed "selfish" will negotiate and come out ahead and earn the respect of those too afraid to ruffle feathers by asking for or demanding what they want or need in compensation.