Mischief Maker wrote:

Skyler White is not the love of his life. Gretchen Schwartz was. Skyler was the next runner up.

Walter is PASSIONATE about science. Skyler doesn't share that passion, Hank doesn't share that passion, Walt's students didn't share that passion, and even Walter Junior doesn't share that passion. Gretchen Schwartz did. Walter's flashback to an evening where he and Gretchen map out the chemical composition of the human body was one of the most romantic scenes in the show. Compare this to the rather pathetic story of Walt's courting Skyler via crossword puzzles.

At some point Walter broke up with Gretchen and it was UGLY. Walter was so pained by the breakup that he wanted no contact with her at all. When she got together with his business partner at Grey Matter, Elliot Schwartz, he quit the company to keep her away.

Look at the episode in Season 1 where he and Skyler go to Elliott's birthday party. Skyler is intimidated by her poverty verses all of Walter's wealthy old colleagues, but Walter isn't intimidated. The other scientists remember his achivements and have nothing but praise for him. While other people are buying incredibly expensive gifts for Elliot, Walter becomes the star of the show by showing them all up with his cheap box of ramen noodles. When Skyler pleads her case to Elliot, he offers Walter a job, and later just to pay for Walt's chemo, without any hesitation, or any hint that the two of them had any bad blood in the past. It's Gretchen, who Walt avoids the entire party, who finally calls up Walt and asks if his refusal to accept their help has something to do with her.

I like to think that Gretchen and Elliot did not actually steal Walt's research, but that that's a story Walt convinced himself of to keep Gretchen away.

At the time of the Pilot, Walt has built a life for himself away from Gretchen, but it's turned out to be a pretty shitty life. He's adrift in a world of friends and relatives he has nothing in common with. He's so miserable and numb he can barely perform when his wife gives him a birthday handjob. Then he gets the cancer diagnosis and you all know how incredibly freeing that all was for him and blah blah blah, but I think he wanted to die long before the diagnosis to escape his miserable alternate life with Skyler and that was the REAL reason he refused Chemo at first.

This is why he didn't Skyler about the cancer diagnosis for months, and why he still hasn't leveled with her about his current predicament. She's not his soulmate and it didn't even occur to him that she needed to know, just that he fulfilled his husbandly duty to provide for her and their children. He doesn't want to hurt her, but she's not someone he's even remotely close with and he doesn't realize how much his web of lies has hurt Skyler until the very end of Season 2. Don't say I don't give credit where it's due, one of the more effective scenes in Season 4 is when Walter gives what seems like a sincere apology to Skyler for all the suffering his escapades have put her through, only for it to turn out to just be a sarcastic idea for a line to trot out in front of Hank.

Then Jessie and the Meth business give an option C in his life. It's pretty obvious that Jessie is the son Walter Junior never was because while he may know fuck-all, his worship of Mr. White for his chemistry expertise taps into Walter's passion for science that no one in his life outside of Gretchen's circle ever has. The money is almost a secondary concern for Walt, it's the respect and power his MacGuyver chemistry tricks give him over these dangerous underworld thugs, especially when crazy Tuco hands Walt a wad of money after Walt blows up his office. Gretchen is miles away from that world, and Walter can feed his passion. Witness the birth of his and Skyler's daughter, or collect millions of dollars as reward for his chemistry wizardry? No contest.

Providing for his family is just the excuse he gives to the world and himself for what he's doing.

That's my impression, anyway.