Why was he into her in the first place?

She was a very pretty blonde. Sometimes that's enough for a guy.

I know right, I thought the same! He was fun and it wasn't like he was fun but not working etc and looking after their child, her and being generally supportive! She was so self absorbed and selfish, she didn't seem to make any allowances for him, presumably he did most of the child care while she finished medical school and he married her knowing the child wasn't his for FS!? I just got the impression that she felt superior to him and also that perhaps even though she says she was, she might not have been faithful to him? I read a synopsis or critique of this film that wasn't particularly flattering about Dean, but I actually think that he made the greater sacrifices and had the most to be angry about, he was just a romantic at heart and a kind, good person and she wanted him to be overly ambitious and stressed! I don't know if this has been raised already but I don't understand why she stopped seeing the other guy anyway?

He was a romantic, plain and simple. When he is talking to his co-worker about "love at first sight" he genuinely believes it. He was feeling the right way at the right time and he really liked her face. Usually it's the eyes. While even a single glance can produce some really serious feelings sometimes, it's best to realize that there are many variables that account for these feeling and that there is truly no such thing as "love at first sight".

I think it's safe to say they wouldn't have ended up together for the long term if she hadn't been pregnant so early on. Dean might have tried to stick it out since he was idealizing her, but she wouldn't have "put up" (or at least what she considered putting up) with his flaws if he hadn't become the father to her child.