Since I'm not a very eloquent person, I'd like to present an interesting post written by Ravanne which points out not mentioned before issues :
The idea that some Lisa fans keep floating around, that Lisa is meant to be Dean's "end game" and that he will end up with her when the series wraps up is one that I find really frustrating. Even if you had an emotional investment in Lisa as a character and her relationship with Dean, a bit question has to loom. Just how long is Lisa supposed to sit around and wait for Dean to figure out what he wants in life?Source
Seriously... this whole senario of Dean being this helpless creature buffeted by the winds of fate is just grating on my nerves. Dean is a grown man, folks! An emotionally damaged one with more issues than the archives at National Geographic, but is is an adult. He is capable of making decisions for himself. If he wanted to stay with Lisa, or to find a real balance between having both a domestic life and being with his brother, he certainly didn't fight for it. He wasn't trying to convince Lisa to give it another chance, or to explain what happened. He just accepted her decision to end things.
When he accepted her decision, without trying to change her mind, that was making a choice. His rational was weak - that he's not worthy of being with her and he'll ruin her and Ben. But he made a choice. If he wanted to be with Lisa as much as the LisaShippers insist, why didn't he fight just a little bit? Instead the whole thing was simply played out as a reason to give us another double-dose of MopeyDean with his personal Black Raincloud of Saddness over his head.
And as for Lisa... whether you like this character or not, she is entitled to have a happy life, whether with Dean or in another realtionship. How long is she and Ben supposed to put their lives on hold until Dean gets his act together? They never confirmed Lisa's age in the show, but I always got the impression that she was a few years older than Dean. At the youngest, this is a woman in her mid-thirties now and time isn't standing still. Her son is going to be a teenager before long. Their lives can't stop just because Dean can't figure out what he wants to do with himself. If Lisa meets someone that can make her happy and give her and Ben the lives they want, then great for her! Ben's already spent most of his life without a father figure - should he have to wait until he's a grown man himself before Dean comes to the decision that he wants to take Ben to Little League games? At the rate Dean is going, Ben will have kids of his own by the time Dean figures things out.
How long should Lisa have to wait for Dean? It already has been given the impression that she's waited at least since TKAA for Dean to come back into her life. Should she put her life on hold until she's old and gray and maybe Dean will decide to be with her? That's not fair to her. I've never hidden my dislike for her as a character, but as a woman the idea that any female should be expected to sit around and wait because she might be good for a man when he finally gets his act together... the word for that is "pathetic".