calling a callback function with () – Blancer.com Tutorials and projects

I don’t understand why a callback function defined with () is only executed immediately and never again. For instance:

<body>
    <button type="button">Click Me!</button>

    <script>
      const buttonElement = document.querySelector('button');
      function sayHello(e){
          console.log("hello")
      }
      buttonElement.addEventListener('click', sayHello());  // function sayHello will be executed imidiately only once
      buttonElement.addEventListener('click', sayHello);    // function sayHello will be executed only when button is clicked

    </script>
</body>

Why does the first EventListener will be executed immediately and never work again ?