The existential angst which Kierkegaard talks about can be low level but it permeates everything because it is inherent in our having freedom. Not knowing what life will bring us next can be exciting but also terrifying and that gap is what keeps the human wondering and sometimes stressing.The longer we live, the more we witness variability and uncertainty and indeed, as young as Kierkegaard was when he died, his five siblings passed away before him. Those events undoubtedly had deep impact on his psyche which then turned into voracious writing and exploring what we must do to mitigate that longing whose source we cannot adequately put our finger on. The older we are and the less our distraction with child rearing or career aspirations, the more we are faced with the time to reflect on what came before in our lives and what it all meant which can bring its own kind of angst.
The good news might be that this longing can be channeled into what brings us joy and rejuvenates us by working to improve the lives of others and in so doing perhaps improve our own.