The problem with existentialism in my mind is the idea that all life is that nothing matters except for the fact that life is innately valuable. I strongly disagree with this premise as there are gaping holes in such an argument. Surely the life of Rosa Parks or Martin Luther King Jr. has more value than the life of Osama Bin Laden, or any of the most recent school shooters. If life is deemed valuable for the sake of being a life there is no morality, and with out morals we are no different from animals.
Our yearn to build is what makes us human, and the need to do it together is what puts us above other species. It is the friendships and relationships we form over time that make life worth living, and it is existentialism that deems all of that worthless.