Despite everything Barry's (Grant Gustin) done over the course of the first two seasons of The Flash (and despite being the fastest man alive) he's still a little slow on learning the value and importance of the surrogate family he has with the Wests. Which I know is a little weird since he and Iris have all the lovey-dovey eyes, but my basic point, and the show's, is this: Barry will always return to doing whatever is best for the Wests — and the Season 3 premiere "Flashpoint" proved that once again.
Spurred on by the events of the penultimate episode of Season 2, Barry zipped back in time in the Season 2 finale to save his mother from the Reverse-Flash — and in doing so created an alternate timeline. "Flashpoint" opened 3 months into Barry's experiment in time travel. He was leading the good life, living at home with his still-alive parents and still working as a CSI at CCPD. All that was missing from this idyllic re-shaping of reality was a relationship with one Iris West (Candice Patton), who barley remembered him from elementary school. Luckily, it didn't take much to get that started, because Barry is totally charming and goofy.
As he and Flashpoint-Iris grew closer together, Barry was drawn into the battle between the Flashpoint-Flash and an evil speedester, The Rival (there's always another speedster). This timeline's Flash was, naturally, Wally (Keiynan Lonsdale). Meanwhile, Flashpoint-Joe (Jesse L. Martin) was a drunk who couldn't be bothered to show up to work most days, and he didn't much care for the fact that Barry was taking an interest in getting him to work — or taking an interest in his daughter. Basically, in a matter of only a day or two, Barry had, after three months away from them, embroiled himself in all the Flashpoint-Wests' affairs.
This isn't a shock, of course. While Barry had the excuse that his Earth-2 counterpart was married to that Iris, Barry did spend a chunk of his time on Earth-2 last season trying to make lounge singer Joe actually like Barry instead of, you know, saving Jesse from Zoom. Nor was it a shock that as Flashpoint began to settle into place — if there's a single consistent rule to time travel in the Arrowverse, it's that alternate timelines take time to solidify — and Barry's memories of his old reality began to fade from his brain, including the OG Iris, that Barry did was he could to save the Wests from the fate he has created for them. Wally was risking life and limb as (Kid) Flash and, by the end of the episode, was on death's door since his powers weren't healing him as they should have. Joe, already in the throes of alcoholism, would've likely crawled deeper into a bottle. Iris would have likely lost both her brother and father because of Barry's time travel therapy/denial.
Barry could not allow this. Remember that Barry is a man who was wracked with guilt over the death of Earth 2-Joe, and that guy hated him. It didn't matter, however, because he was still Joe West. No matter what reality he calls home, Joe West is Joe West to Barry. This is why Barry's decision to undo Flashpoint, his "reverse-It's A Wonderful Life," isn't surprising.
Instead, it's an affirmation of the singular importance that the Wests have in Barry's life. He is as drawn to the Wests as he is own parents, and for good reason. The Wests are Barry's constants, the people that help to anchor him regardless of whatever alternate Earth or alternate timeline he's currently existing in (unless it's Supergirl's Earth, and then it's hanging out with Kara and eating all the dumplings). They're as much as a part of Barry's sense of self as his parents, dead or alive, are. He is incomplete without them.
And so Barry sacrificed his own happiness for the Wests and allowed Reverse-Flash to once again kill Nora Allen and set right what Barry had put wrong. It would, to Barry's thinking, fix everything. Yes, his parents might still be dead, but at least his surrogate family, the one that has been there for him time and time again, would be whole. For once, however, mucking with time actually had at least one side effect: Joe and Iris are estranged for reasons currently unknown. And it's all Barry's fault.
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