The reason is a simple logic error on your part.

When you create a new tag, it only lives in memory until you save it. Hence the tag won't be assigned an id because it hasn't been saved. Once you save the new model the auto_increment on the id field will generate a new id and laravel will assign the id to the model.

The tag needs an id to attach the user to in the lookup table.

Try this:

$tag = New Tag;
$tag->name = $tagName;
var_dump($tag->id);

^^this will throw a error because tag has no id yet^^

Now this:

$tag = New Tag;
$tag->name = $tagName;
$tag->save();
var_dump($tag->id);

^^this will display the new id^^

So what you want is

$tag = New Tag;
$tag->name = $tagName;
//save tag to get a generate a tag id on the model
$tag->save();
$tag->users()->attach(Input::get('userID'));