Well, I've never been exactly sure, but I think it has something to do with the loss of hearts. I know it's not care misses (where your tamagotchi calls for attention and you ignore it), since I've tried getting care misses several times and I've always gotten perfect care tamas. However, when I got my new V6, I connected it with my older V6 frequently. The new one lost a lot of happiness hearts since she had lower musical skills and lost most of the games played with the older one. She evolved into Violetchi. I've experimented with this, but I either get an okay care or a perfect care character. I'm thinking it may be based on a ratio of hearts gained to hearts lost, but I'm not entirely certain.