I'm fairly sure she or he explained real briefly in passing once that he up and left once, she came from money and he didnt and felt that he wasn't good enough for her parents so he left and that was that. I'll need to do some sleuthing.
No; he never said this exactly. However, there was a conversation between him and Gratchen where they both recalled events very differently. She said "we were on vacation with my parents at the shore house and then you left without a word, you abandoned me". Which made Walt blow up. There were some subtle clues that Gretchen and Elliot came from wealth, while Walt was probably there on scholarship but nothing concrete--but that's where the whole theory came about that Walt might have been told he wasn't good enough for to marry Gretchen.
There were other theories that Walt's pride took something way to far. But we don't know--the writers don't even know, because they literally never wrote the back story, which they admitted in an interview. They weren't even going to have anything about Gray Matter in the final season until the Cancer Kid told them that the fans felt it was kind of important.
Walt's pride and how it fucked him was an important part of the show. I do agree with how stupid that decision was though. All throughout the show people were hoping to see the backstory behind Walt selling his share of the company, partially because it would make that decision to reject a job more believable.
Honestly, them NOT telling the back story; brought the show down a notch for me--but I understand it. The fact is; I'm guessing even the writers knew that no explanation was going to do the mystery justice so they avoided it--because no matter HOW you explain it, it just doesn't make sense. So many holes come up. Being part of a noble winning team; having your name associated with billion dollar patents, these are things that can get you a job literally anywhere, no problem. If he had those credentials and the ego-maniac pride they implied then the start of show becomes kind of absurd. Long and short, his trajectory after Gray Matter makes absolutely no sense if we believe Walt was always like he was; because no way does he take a defeat or a "slight" by Elliot and Gretchen and not attempt some kind of reprisal (By making a name for himself elsewhere--and given those credentials he'd have doors open for him everywhere from DoW on down.)
And yet, if he wasn't like that--if his pride wasn't always a problem, then he'd have taken the Gray Matter job no problem. Him NOT taking the job becomes the absurdity, him not taking charity becomes kind of absurd. Either way; I think they knew, once they opened that Pandora's box and attempted to explain it? It would have diminished the arc of Walt in the show (Going from soft spoken, super-polite man to a drug lord killer.) But still, it bugged me not knowing and it does feel like there is a hole in the show due to it.