Why does my dog roll on her back?

When a dog presents her belly and neck in this situation, she's expressing an ancestral trait rooted in wolf pack behavior that was used to communicate deference to other “alpha” dogs.

There are many reasons a dog may expose hi/her belly.

1) Most likely, the pup is inviting attention or affection (belly rubs, etc) or inviting you to play. This is usually accompanied by a relaxed demeanor, including relaxed muscles, tail not tucked between the legs and typically wagging.

2) The pup is anxious but social. Unlike anxious avoidant dogs that will try to avoid people entirely, typically this dog will approach you in a cowering fashion (head low, sometimes shoulders low and somewhat crouched in their approach) and then roll over onto their back right at your feet. In this way, they are establishing you as the one "in charge" and showing you that they are no threat and mean you no harm. The telltale sign for the anxious but still social dog is a tense, non-aggressive approach coupled with a tail that is tucked between the legs when it rolls over.

3) The pup's trying to cool off. I've noticed this in our dogs when we come back inside after being out on a hot day. Our younger pup will literally fall asleep on her back with her legs up in the air. While dogs typically curl up to lie down, they're more likely to sprawl out if they're in a warm environment. You'll also see dogs rest on cooler flooring types (tile instead of carpet) on a hot day.

Because she trusts you! Although it's considered a submissive behavior, pups will only show their bellies if they are comfortable with the person. So if your pup rolls on her back for more belly rubs, it's because she trusts you :)!