Why my function with sending message from popup.js to content.js is executed immediately when clicking on extension?

Question

My goal is to execute function with sending message just when user clicks the button. Instead of this, function is executed when user clicks extension icon in toolbar, why?

code from popup.html:

<button id="selectTagButton"></button>

code from popup.js:

let selectTagButton = document.getElementById('selectTagButton');

selectTagButton.addEventListener('click', buttonClicked());    

function buttonClicked() {
  chrome.tabs.query({active:true, currentWindow: true}, function(tabs) {
    chrome.tabs.sendMessage(tabs[0].id, {buttonclicked: "true"}, function(response) {
      console.log(response.selectedtag);
    })
  })
}

code from content.js:

chrome.runtime.onMessage.addListener(
    function(request, sender, sendResponse) {
        if (request.buttonclicked == "true") {
            let tag = "abc";
            sendResponse({selectedtag: tag});
        }
   })

Am I missing something or I'm just doing it in wrong way?