I know there's plenty of discussion around how Ruby imitated Dean to appeal to Sam, but I've been thinking about how similar to Dean she actually is.
The parallels between Dean's devotion to John and Ruby's devotion to Lucifer, who is like a father figure to demons. Dean is conpletely loyal and obedient to John, and Ruby is completely loyal to Lucifer. Dean spent most of his life on his father's mission, and Ruby spent a significant amount of time in pursuit of Lucifer's freedom. They both suffered immensely for their cause, and they were both outsiders - Dean from human society, Ruby from demon society. They're both ruthlessly motivated, loyal, and willing to endanger themselves or even die for their cause or the ones they love - Dean dies for Sam, and Ruby dies freeing Lucifer, and she had to have suspected that it would end like that for her and still went through with it.
On top of the way she manipulated Sam and took advantage of him during an extremely vulnerable time, it's no wonder Sam was drawn to her.
