박스안에 채울 때 저랑 튜터님이랑 같은 코드를 쓴 것 같은데 왜 저 박스는 서로 붙어있을까요..?

<div class="mytitle">
    <h1>십센치(10cm) 팬명록</h1>
</div>
<div class="box">
    <div class="mov_url">
        <input type="text" class="form-control" id="floatingInput" placeholder="닉네임">
    </div>
    <div class="form-floating">
        <textarea class="form-control" placeholder="Leave a comment here" id="floatingTextarea"></textarea>
        <label for="floatingTextarea">응원댓글</label>
    </div>
    <div class="mybtn">
        <button type="button" class="btn btn-dark">응원남기기</button>
    </div>
    </div>
</div>