Because they have great chemistry.
Because they are so not over.
Because they never really stop loving each other.
Because she has a hot dream about Red and her kissing and more haha when she is engaged with someone else.
Because even though you can tell he was devastated Red congratulate Vanessa for her engagement.
Because he and his football players went to cheer up vanessa's cheerleaders.
Because they never stop flirting when they are together.
Because she didnt told him, she was in a relationship, Derek did.
Because he follow Vanessa when she said it was the first time they met even though its not true.