That feeling caused by intense longing, nostalgia or heartwarming which is really intense and feels like someone or something has reached into your chest and is squeezing your heart.
"Terry gets massive heartsqueeze every time she sees photos of her old cats."
(1) Did you watch that really famous super sad movie?
(2) Yeah. Still can't get over dat heartsqueeze.
When you're in a good mood and decide to always add in hearts to a conversation. The person talking to you might copy you, and start a cycle of just ending off every message with a heart.
Ron: I'm feeling so heartsy! <3
Jen: You're such a loserr!!! <3