Reasons:
01] Because everyone secretly loves to watch step-siblings hook up.
02] Because Casey was jealous of Emily.
03] Because Derek was jealous of Sam.
04] Because their fighting is just built up sexual tension.
05] Because they look good together.
06] Because they both know and see each other's true selves.
07] Because she sees the good in Derek.
08] Because she was waiting for their feel good moment.
09] Because Derek needs Casey just as she needs him.
10] Because Derek is the exception to all rules.
11] Because there are exceptions to all of Casey's rules.
12] Because she planned his birthday party.
13] Because "Your force-field is activated."
14] Because he activated her force field.
15] Because "she's perfect".
16] Because George and Abby were going to send Derek to Spain
because he kept getting distracted and doing things to Casey.
17] Because nobody talks to his step sister like that except him!
18] Because they have eye sex all the time!
19] Because she dragged him into the girl's bathroom.
20] Because she protected him from a bully.
21] Because a lot of Derek's home movies star Casey.
22] Because they have so much UST.
23] Because he took her to prom.
24] Because he stays up all night thinking about her.
25] Because she is always screaming his name.
26] Because he made a birthday video for her.
27] Because Derek seemed a little too happy that Casey was wearing his shirt.
28] Because Casey is always pushing Derek onto his back.