And we think that's just what happened to Millie Bobby Brown.
The Stranger Things star just turned fifteen, and she posted a celebratory video of herself holding a '1' and '5' balloon on her Instagram.
"[You] have such big heart," Ariana wrote on the photo, continuing by saying, "anybody who knows u (even just a little) is [very] lucky indeed. i'll jus be here loudly reminding u of that whenever u need me! hbd ily."
"@arianagrande thx ur the best doppelgänger a girl could ask for. ilysm calling u now lol," Millie replied.
Living for this friendship.
And the b'day love didn't end there! Millie got adorable messages from other celebs like Dua Lipa, Lauren Jauregui, Paris Holton and co-star Caleb McLaughlin.
Watch below... Fans think Stranger Things is ending because of Millie Bobby Brown's tearful Instagram post: