Man, fandom seems to be split on this, as I'm discovering. I think more people dislike it, but that could just be the part of fandom that I'm seeing. Anyway. I liked Lisa fine in her S3 ep, and even in the dream ep. Just another girl Dean has hooked up with. Hey, he even liked her. But that seemed to be where it ended. So to have Dean talk about her like she's the one, yeah, I don't get it. It did feel forced.

So nothing, nada, just like with Cassie and all the other girls he's been with

This, too. I've seen Cassie's name brought up as "why wouldn't Dean think of her and not Lisa?", but I also see nothing there. Yes, he had a relationship with her, but that doesn't make her the love of his life, either. A few weeks, wasn't it? Certainly nothing longer than a month or two. But all of the girls on the road end up being left behind by default. And you know, I kind of like it that the show doesn't try to shoehorn in long term romance, because it really doesn't fit with their lives. And not that I really liked the idea of Dean/Jo, but Show, if you wanted to hook him up with someone, there was someone he'd seen more than once, and who he didn't have to keep secrets from. But wait, you killed her off.

Lisa and Ben came to represent what he wanted but couldn't have.

Yes, this. I have no problem with them as the representation. But when he shows up on her door talking about her like they'd actually had something, just no.

Hell he even said that he's never been in love in 5.15 (which surprised me since he supposedly was in love with Cassie at some point at least)

True. And true. Although even if he has been in love, that doesn't mean he's still in love. You can love someone and move on.

the writers are trying to sell me a much deeper relationship than what's there, or at least what has been portrayed on the show

Yeah. If we had seen more of Lisa, or seen her since S3, or if Dean had made reference to her any other time, this would make some sense. It just seems to be trying to turn a fling into something deeper without any development at all. Also, I can't help but feel that the writers just scurried around, asking, "Who can we bring back here, even though we concluded her story in the episode she was in?" It's not like Lisa even had a huge part to play. Really, the ep she was in was mostly played for laughs (besides the ever present angst of Dean going to hell that season). I never once seriously considered that Ben was going to end up as Dean's son, nor did I think Lisa was lying to him or letting him off the hook.

What's really bothering me though, is the thought that that it may play a part in the overall storyline in season 6; to have Dean settle with her and Ben.

OMG. O_o I hadn't even thought of that. Please, no, please, no.