Good question. (And if you haven’t read Part 5 of my ongoing Season 7 analysis yet, you can read it here.)
No, I don’t think Lydia is a rebound from Lisa, because I don’t think Dean was ever in love with Lisa, an idea that Jensen Ackles himself supports. He was in love with what she represented to him: stablility, comfort, healing, closure – the chance to live a normal life. After Sam’s fall into the Cage, she represented a family that would never leave him.
Hence why Lisa goes from being the “bendiest weekend of my life” in Season 3 to the ideal of everything he’s ever wanted and never had by the end of Season 5. It’s not poor or inconsistent writing. It’s an illustration of how Dean’s perspective on Hunting changes after Hell. Before Hell, he assumes he’ll never be able to leave the life, and he doesn’t think to let that bother him. After Hell, however, he resents Hunting and everything it represents; he thinks that if only he could leave the life, he might have a shot at real happiness.
It’s a common thing you see in veterans or trauma survivors, where they idealize a “normal life” and the people who live it until they’ve convinced themselves it’s pretty much perfect. But nothing, and nobody, ever is.
Lisa comes off as a bit of a doormat in Season 5 and especially Season 6, but I think that was intentional, because to a great extent, Lisa has idealized Dean as well. For saving her life and her son’s in Season 3, he has become her hero, her knight in shining armor. She’s even idealized him to some extent in “The Kids Are Alright”, apparently telling her friends about “The Dean”, and that that their brief fling was the “the greatest night of my life”. Thus when it becomes clear to Lisa in “The Kids Are Alright” that Dean is every bit worthy of the pedestal she’s placed him on, she radically alters her behavior in Season 5 and 6 so she doesn’t chase him away. She behaves submissively, non-threateningly, as one would to a wounded animal. She doesn’t question his decisions. She doesn’t argue with his orders. She completely changes who she is, just because she’s terrified he’ll run away again forever.
Which, of course, he does anyway. And when Dean leaves with Sam, she knows it’s the end, but she holds on as long as she can, takes whatever she can get, because that’s what you do when you’re afraid you’re losing the idea you’ve built your life around. I believe that Lisa always suspected Dean would leave eventually. As a single mother from a broken family (which I think is subtly implied in the way she used to sleep around, and how vehemently she reacts when Dean shoves Ben against the wall), she probably felt she deserved nothing less. And once Dean leaves with Sam, notice how Lisa begins to regain her backbone. She’s not afraid of losing Dean anymore, because she’s already lost him, and she begins to reclaim the independence we saw her exhibit in “The Kids Are Alright”.
I have no doubt that Dean loves Lisa, or that Lisa loves Dean – but it’s not a romantic love, no matter how badly both of them want it to be, because how could it be? They have nothing in common, they share no hobbies or shared interests. They don’t seem to be able to communicate or have faith in each other. All they share is physical proximity, and once Dean leaves, that’s gone too.
Season 6 had to happen. Dean had to live with Lisa, and the scales had to fall away from both their eyes. For Dean and Lisa both to grow as people, they needed to have these illusions dispelled. But that doesn’t mean it wasn’t going to be painful.
Frankly, though, I think the whole storyarc might’ve gone a little over many of the fans’ heads, because the Supernatural fandom does skew young, and a story about fatherhood and commitment is inherently more adult. That’s not a knock on younger fans by any stretch, but I do think that Dean and Lisa’s mistakes in Season 6 don’t really make sense without the benefit of experience and a few failed relationships under your belt. To a teenager, a story about becoming an adult (Seasons 1-5) is far more interesting and relevant than a story about what you do once you become one (Seasons 6-7).
Anyway, hope that answers your question, nonny. :)