After season 5's finale where Sam, possessed by Lucifer, went to hell and was inexplicably back on Earth, Dean attempted a normal life, as he promised Sam he would. At the start of season 6, audiences saw what Dean's life was like without hunting or Sam — for a year.
However, when any sign of his old life crept up, his original angry tendencies slowly returned, like when he lashed out at Ben for holding a gun. Many actions, admittedly, revolved around one of Dean's biggest fears in S: Endangering his loved ones. He had numerous conversations with Lisa about this in an attempt to make rational, mature decisions. If Dean hadn't erratically moved Lisa and Ben to Bobby's when strange monster-related occurrences started to help Sam with hunting, he wouldn't have so easily reverted to his pre-suburban life mentality, which often involved reckless decision-making.
Once Dean completely closed the door on his normal life by asking Castiel to wipe Lisa and Ben's memory, he also erased all semblance of the characteristics he exercised during his time with them.
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