It is the highest-grossing animated film of all time and one of Disney's top franchises. It was the most downloaded movie from Apple in 2014, and kids everywhere are still singing the movie's anthem, "Let It Go."

Heck, it's now probably stuck in your own head right now, even if you've never seen the film.

So why has the 2013 movie stuck around for so long for the under-5 set?

Experts say it is not just because you can find images of the movie's sisters on everything. Elsa and Anna are on dresses, on scooters and even on jelly beans.

The news that a long-awaited sequel is finally coming in November 2019 was met with cheers (and a few groans) across the Internet.

Frozen 2 is coming to theaters November 27, 2019! pic.twitter.com/iW4JR2RSfm — Disney Animation (@DisneyAnimation) April 25, 2017

To understand the psychology behind "Frozen" Mania, in 2015, CNN reached out to psychologists who are sisters themselves: Yalda Uhls is regional director for Common Sense Media. Maryam Kia-Keating is an associate professor of clinical psychology at University of California, Santa Barbara. Here is our edited conversation.

CNN: Princess movies have been around since the beginning of time, but this has really resonated. What has made this one so unique?

Yalda Uhls: One of the things that really struck me, and I think struck little children, is that there is a really strong intergenerational, family-themed message here. Despite the sisters Anna and Elsa being separated for so long, the story is ultimately about the bond between the two of them.

When you're little, that is your zone; that's your group; they define your world.

Highlighting that and making it feel important -- your sister, your brother, your mom and dad -- is something little kids can really identify with.

Maryam Kia-Keating: Kids can really understand the idea of their sister wanting to play with them, and sometimes not, but still having a strong family bond.

Ultimately, it's about the love between the sisters. That's a message that many little children understand. There are also good lessons about overcoming struggles and facing life challenges. But, what's interesting about preschoolers, in particular, is there's this loyalty and unrelenting interest to watch this movie over and over again.

Part of it is because it was so well done, but there are some themes that younger kids can really identify with.

My 4-year-old daughter told me that she also liked that it didn't have a witch.

I wouldn't have thought that until she said it, but it made me more thoughtful about all the other movies -- great movies -- that have scary witches and themes in them.

This was something that little kids under 5 aren't going to have nightmares about.

CNN: Why is not having a witch important to a preschooler?

Kia-Keating: When you're an adult, you've heard a lot of these stories, like "Snow White" or "Cinderella," and you forget the power they can have.

It's helpful to have a child's point of view on this.

Kids are living in this world where the line between what's real and what's not really is blurry, and because they have such vivid imaginations, witches and monsters can feel very real.

As adults, we may see these villains or monsters as a way to tell a story or a lesson, but when children are in the moment of being scared, they are caught up in the fear and not in finding the larger lesson.

Uhls: I used to work in the film industry, and as adults, we're told that we need conflict to drive drama. Take Grimm's Fairy Tales; there are always dire stakes. Or movies like "Bambi" and "The Lion King," where the mother or father die.

These are serious issues and themes, and sometimes little kids aren't ready to process and understand these ideas.

CNN: What about the characters seem so easy for kids to empathize with?

Uhls: Preschooler imaginations are really strong, and so they respond really well to stories with magical realism.

In "Frozen," Princess Elsa has these powers to control ice and snow -- and that really captures kids' imaginations.

When you're small, you don't feel powerful. Everyone is always dictating what you can and cannot do. Having a character they can empathize with that has these powers can make them feel more empowered, as well.

Kia-Keating: That magical piece is exciting and is one of the things that really sets this movie apart from others.

Another aspect that kids see themselves in Elsa and Anna is the impulsiveness.

Parents are constantly trying to teach their little ones how to self-regulate their emotions, how to behave, how to sit down.

Elsa is constantly trying to tame or control her emotions -- and while this is something that is salient to everyone throughout their lifetimes, this is something preschoolers deal with all the time.

CNN: The song "Let It Go": It's everywhere, and kids can't stop singing it. What is it about this anthem?

Kia-Keating: My 4-year-old came home and learned the song before we had even seen the movie.

One of the lines she and all her friends connected to was "Be the good girl that you always have to be."

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And when they sing it, they wag their fingers like they do in the movie.

I think it looks a lot like something they see and hear from parents -- be a good girl or boy, don't do this or that -- so part of it is copying what they frequently hear.

But when I asked my daughter what she thought the song was about, she said it was about "Elsa being happy and free, and nobody bothering her."

That's a message that everyone wants: to be happy and free.