>implying they do
Nobody lives for themselves. People serve each other. It's why the economy works, why labor relations can be maintained. And why people are responding to your thread and why you're reading their replies. People love each other, no one is alone. Even those who like to say "I think for myself" still accept high degrees of outside information. For example, when solving a Physics problem, you choose to believe in a formula that you read in a book (instead of figuring out the Laws of Physics from scratch all by yourself, and even then you'd still be using Math to do so). Only nothingness can be described purely recursively, in terms of itself. Everything else can only be defined in terms of other things, at least in part.
Humans are not rational. They're not "axiomatic theorem provers". Your type of rational thinking can only lead to a spiral of madness, lonesomeness and mental pain and the sole escape will be psychopathy. If people cared about their lives, there would only be psychopaths predating on each other, seeking the best for themselves, without a drop of pity, compassion or empathy for others. There would be no laws, or at least no one would truly believe they should be followed. The Japanese wouldn't be drawing cute anime girls for fun and would instead devote all their time to work in their giant power hungry companies ruled by strict Confucian ethics. The world would be gray, with colossal skyscrapers but no parks.
As transhumanist technology becomes more advanced and widespread, more individuals will stop caring about everyone else. People will want to escape the constraints of humanity. Humanity and its ideals will lose form. They will say "humanity never made sense in the first place". I think, you stop being a normalfag if you have the courage to take this leap.
If you do care about your life, you should only seek knowledge and meaning by interacting with other beings. Eventually, either you'll become some sort of god or kill yourself.