|Red dots signify happiness|
The thing is, when we meet Walter he already has quite a lot of history behind him, something that really supports his actions and determines who he is. The flashbacks in the show just weren't enough. I'll try and point out what should have been dealt with during the making of the show.
So, imagine Walter, a young, successful chemist, with so many roads ahead of him, and a vision of the future that seems quite bright. He has a girl he's in love with, Gretchen, and he joins forces with his friend and colleague Eliot Schwartz to make Grey Matter Technologies. All seems well as we see Walter and Gretchen brooding on the components of a human being. However, there's a problem. Walter is not from a wealthy family; all he has he created with his own hands, using his brilliance. That is thrown in his face when he is to meet Gretchen's parents, who Walter sees as belonging to a higher class than himself. This thought troubles him (one thing you notice throughout the regular show is that Walter is always aware of his social status) to the point that he leaves Gretchen and the company he created. He feels out of place there and needs to run away. Now that must be quite a shock for him, especially when, later, his pride kicks in as he sees the company he left flourish and become highly successful. It certainly doesn't help that Gretchen, his former love of his life married Elliott, his former friend. All of this basically kills a part of Walter. He is torn into pieces and seeks comfort by recreating himself as a man who assimilates. That way he won't get hurt again; you can't fall down if you don't even try to fly. So he settles in a small home, with a small job, and a small family, far away from his previous life, far away from any glamour. Just when it seems enough time has passed, after he has submitted himself to the uneventful life, he gets cancer and his world starts falling apart. Everything comes back to him, all he once had, all that could have been, all that he is now - nothing that he will become!
Had we been acquainted with all that we would have realized how understandable Walter's further actions were. The glare in his eyes when he finds a new way of becoming successful (and by using his own hands and mind!), not wanting anything from Gretchen and Elliott, ever again, and all of his later re-awakening.
I know this first part probably wouldn't attract that many viewers, but I do believe it is necessary for understanding most of the show, and basically for giving it some depth (and quite a depth it is).