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You start to notice that they understand each other, that they fit together perfectly, that they're exactly what the other needs.
That's why people really ship them, why people believe in this couple.
Derek/Casey could be considered the fandom's OTP, as nearly all the fans like the pairing even when the network & creator's best efforts in the contrary.
I don't buy half the crap you tried to shovel us in season 4. It all seems like our opinion is trying to be swayed in an alternate direction (i.e.
"This season: Casey starts dating the captain of the football team and finds her “cool” status at school rise as a result; Derek meets the girl of his dreams; Edwin gets his first girlfriend; and Lizzie finally admits to liking a boy.
They also reveal that they do actually read the stories as well. Disney and the creator, Daphne Ballon of the show are strong opponents to the fans and this ship (more than likely because of the they believe the pairing incestuous).
Because of this they often add in "significant" others for Casey and Derek each which causes anger and displeasure in such storylines of the series.