I have always been told that the sentiment of putting oneself first is selfish. We often hear stories of a parent wanting to work and put their children in daycare. Those parents are typically referred to as "selfish". The modern day young women who want to have a career and not a family is considered "selfish". Those who quit their job, break up with their significant other, or decides to put themselves first is not selfish. Those people are making the decision to focus on themselves, that is important.
I have been told that this idea of putting oneself first, is a "modern one". That "back in the day" you did not have that option, you did what you had to. My response to that, "did that make you happy?" I think we all put our happiness on the side burner when it comes to our professional and personal lives. I think that we need to find a happy medium between happiness and reality. No one is going to do whatever they want all the time. But, to put yourself first one and a while is important.
The simple things are typically the ones that make the biggest impact. Giving yourself the night to catch up on the things you love to do. Whether it be watching television, movies, reading a book, cleaning, cooking, or relaxing. Putting yourself first betters the person who you are. By giving oneself time, and thus putting them first. You are building your creativity, work capabilities, motivation, and drive.