1) You see someone extremely hot.
2) Someone you love does something really sweet or good to you.
"Last night, I melted when my boyfriend did the sweetest thing ever!"
He's not coming, he's staying in with his bird.
Dean is such a melt!
It's called melt because it melts your emotions into a puddle of depression like on a grey rainy day. The facial expression whilst melted is often sad and lifeless. Then you say your face is melted. You also say your face is melted if someone is lecturing in your face and it's boring as hell.
Also note your body language, such as slouched shoulders, a tilted head, and general lack of enthusiasm, it kinda looks like your body is trying to melt, and your emotions feel melted too.
Variants: Melts, Melted, Meltdown, Meltworthy, Melting, Melthead, Melting Faces, Melting Boats (boat-races is rhyming slang for faces).