When you make a commitment to someone specific and form a relationship, you are committing to including them in your life as you. It is the souls' quest for oneness. And because of this, you include their happiness in your happiness. Now, taking action to ensure their happiness (as long as doing that does not destroy your own) IS taking action to ensure your own happiness. There is no contradiction whatsoever then between making someone else happy and making yourself happy. Making someone else happy in a relationship does not feel like a sacrifice if seeing them happy makes you happy. Which is what makes it not an actual sacrifice.