The answer as to why we should be more selfish is similar as to why we should be altruistic; the first answer is because not everything revolves around others, and the second answer is, because not everything revolves around yourself.
Therefore, because both answers are true, we should be more selfish the same as we should be more altruistic, because there isn’t a one entity, whether yourself or others, which is being revolved by the universe.
By being more selfish, we can prevent people from walking over us, and by being more altruistic, we can prevent guilt from the possibility that we have mistreated and/or underestimated others.
Thus there are times when we should be more selfish, and times when we should be more altruistic.
When interacting with someone you don’t know on the internet, for example, you should be more selfish, because that person might as well try to deceive you into paying them by telling you lies that’ll make you worry about them. Such occurrences have happened before and, unfortunately, they might happen in the moment you’re reading this article.
Here’s another example to the opposite state. When taking care of a baby you should put their needs over yours, because they are way more dependent on the one responsible for them than you are on them. Since they need help in basic functioning, a functioning that you are not dependent on them in order to occur, then logic dictates that you should put their needs over yours, assuming you have responsibility on them.
Being more selfish and more altruistic don’t have to be two things that contradict themselves. With enough judgement one can decide when to be more of the former and when to be more of the latter.