Why does a div change the way my content is displayed?

<header>
            <nav class="container">
                <h1 class="logo">LOGO</h1>
                  <div class="menu">
                        <div class="item-1 item">Item 1</div>
                        <div class="item-2 item">Item 2</div>
                        <div class="item-3 item">Item 3</div>
                        <div class="item-4 item">Item 4</div>
                        <div class="item-5 item">Item 5</div>
                        <div class="item-6 item">Item 6</div>
                  </div>
            </nav>
       </header>

* {
    box-sizing: border-box;
}
body {
    font-size: 1.35em;
    font-family: 'Varela Round', sans-serif;
    color: #fff;
    background: #e8e9e9;
    padding-left: 5%;
    padding-right: 5%;
}
.container {
    padding: 10px;
    background: #fff;
  border-radius: 5px;
    margin: 45px auto;
  box-shadow: 0 1.5px 0 0 rgba(0,0,0,0.1);
}

.logo{
    color: #3db5da;
}

.item {
    color: #fff;
    padding: 15px;
    margin: 5px;    
    background: #3db5da;
    border-radius: 3px;
}



/*flexbox*/
.container{
    display: flex;
    flex-direction: column;
    text-align: center;
    // height: 100vh;

    .menu{
        display: flex;
        flex-direction: column;
    }
}