The Contact Form 7 plugin does not add label tags to forms by default. You must add them yourself. In order to do this, open the contact form and add the label text right next to the input field. For example, the default contact form that Contact Form 7 sets up upon installation will have an email address input that looks like this:
<p>Your Email (required)<br /> [email* your-email] </p>
To add a label tag to this, edit the code to look like this:
<p><label for="emailfield">Your Email (required)</label> [email* your-email id:emailfield] </p>