One dictionary definition of selfishness: "Chiefly concerned with one's own interests".
In other words, being selfish is fundamentally about pursuing one's own self-interest. I would ask "Well, whose interests should you be chiefly concerned with?" If you have children, the answer could be theirs, but if not, the answer should be yours.
Here are some of the valuable outcomes of selfishness to which I've been exposed:
A huge percentage of my clients come to me for career counseling because of a fundamental dissatisfaction with the work that they're doing. The dissatisfaction can have many causes, ranging from "I didn't think the work would be so boring" to "I had no idea that I'd wind up earning so little" to "There's always so much work to be done that I hardly get to spend time with my family". But another common cause of people's unhappiness with their career is that they've wound up in their line of work in order to please someone else, or at least in great part to please someone else.
Sometimes this "someone" was (or is) a parent: "Dad was a doctor and it was just assumed that I'd be one, too. He'd have been crushed if I didn't follow in his footsteps".
Sometimes the "someone" is a teacher: "Professor Charan's finance class was my favorite in business school, and he beamed when I told him I was going to work for Goldman Sachs".
Sometimes it can even be a projection: "I need to find work that will allow me to earn well into six figures so that when I have kids I'll be able to send them to the best schools".
Clients wake up at age 30 or 40 or 50 and realize that the career they've been pursuing for so long simply isn't right for them. They might have come to that realization much earlier if they'd de-emphasized the satisfaction they thought they were bringing to others, and focused more on the question "What do I want?".
A common theme I hear from (individual) clients who come to me for guidance on whether or not to end a relationship is the concern that a breakup will be devastating to the client's partner. "I can't break up with her - it would make her so unhappy that I couldn't stand the guilt". Of course many relationships can be salvaged with effective relationship counseling, but more often than not clients enter counseling too late.
In these situations I always ask "Is your partner happy in the relationship now?" and at least 90% of the time the answer is "No". It is rare for one half of the couple to be content while the other is contemplating ending the relationship. Ending a relationship that's terminally damaged is an act of kindness in the long term (even though it may cause tremendous distress in the short term). It frees both parties to pursue connections with other to whom they may be better suited. Pursuing one's self interest when a relationship no longer works is a wise course.
We often agree to do something that we really don't want to do in order to please someone else (or at least get them off our backs). That's perfectly fine, and often necessary to keep relations smooth. If your spouse asks you to take out the garbage or walk the dog and just because you don't feel like it you refuse, you're soon going to be in some pretty hot water. But too frequently we accede to requests and then hold on to resentment for having agreed. This passive/aggressive behavior (silently going along with something but then behaving badly as a result) is toxic.
Before you agree to do something that you really don't want to do, think through the likely benefits AND consequences: "If I agree to going on a cruise for our vacation it'll make my wife happy, but I'll probably be bored and then sulky, which will cause a major fight". You can then attempt to work out a preferable course of action or, if you can't find one, look within to see how you can better deal with a situation you didn't want to be in.