It is the default behavior of most Shopify themes to only show a label next to your option drop-down if your product has two or three options. If your product has only one option, chances are the label is not shown.

That is relatively easy to fix.

Go to Themes → Template Editor.

Now, the hardest part will be to locate the code in your Shopify Theme that is responsible for adding the option drop-downs to your product pages. That code can be in either of 3 files. It can be in your layout file theme.liquid, but you may also find the code in your product.liquid template, and as last resort location in a snippet with name product.liquid. Your theme.liquid file is found under Layouts:

Your template product.liquid is listed under Templates:

Finally, the product.liquid snippet — if you have one — would be listed under Snippets.

In each file, look for this code till you find the right file:

new Shopify.OptionSelectors

Do ⌘-F on the Mac, or Ctrl-F on your PC, to look for the above text.

Found it?

In the one file that contains the code, on the next line after new Shopify.OptionSelectors( ... ), add these 3 lines of code:

{% if product.options.size == 1 and product.options.first != 'Title' %}
$('.selector-wrapper:eq(0)').prepend('<label>{{ product.options.first }}</label>');
{% endif %}

After the edit, your code will look like this:

When you are done with your edit, save your file.

What is the rationale behind that default behavior?

95% of shops don't rename the option label and leave it at Title when they add variants. The Title label is quite unhelpful.