Shortcode Not displayed at the Right Place

I am using custom shortcode to show some message at the end of the post. I am finding it hard to do so because it shows up at the top of the post. Here is my code.

I am placing the shortcode at the end of the post but the message appears at the top of the post.

What should I do to fix this?

function hello_kitty(){
if ( is_single() ){
    echo 'This is hello Kitty message';
}

}
add_shortcode('hellokitty',hello_kitty);