Why is 'this' different when a function is invoked in a callback vs when invoked regularly ex.) showThis();

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ui.title.addEventListener('click', ui.showThis)
ui.title.addEventListener('click', () => {
    ui.showThis();
})

I have a keyup event on an input field in my application that calls a function in my UI Controller module

showThis(){
  console.log(this);
}

which logs the value of this when the input field is clicked.

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Why is it that the value of 'this' is different when the function is called within a callback function on the event listener vs. when the function is itself passed as the callback?