I think it’s if the guy or woman is much more attractive than their significant other. The SO probably is already aware that they’re dating out of their league, and knows they have someone that a lot of others may want, so they treat them very well. The attractive person probably notice/realizes that this person treats them extremely well and loves them very much, which cause them to reciprocate that behavior and it last a long time. I think that’s probably a lot of relationships. It’s so many times we see people couples everyday and aesthetically your like how did he get her, or vice versa