So my theory is that Amethyst and Greg were really good friends and Greg invited her to his concert. He gave her a flyer and was really excited when she said she’d be there. She either forgets/is asleep when it starts, and Rose notices the flyer lying around in their house. She decides to go on a whim, and bam! Greg and Rose fall in love, and Amethyst stops hanging out with Greg because she doesn’t want to get in the way/feels abandoned/resents Rose for taking her friend away.
Amethyst may or may not have had romantic feelings for Greg, but she was hurt regardless. And when Steven was born and Rose died, I feel like Amethyst tried to cheer Greg up/become Greg’s friend again by transforming into Rose, but Greg was obviously uncomfortable about it and asked Amethyst to stop. They get in a fight, and they stop talking altogether (or when they do, it’s as curt and impersonal as possible). That’s why in the “Cat Fingers” episode, Greg seemed nervous/shaken when Amethyst revealed herself to be the cat and started transforming into different things in front of them (would also explain why Steven didn’t know about the the Gems’ transformation abilities in the first place).