You'll have to formulate your own opinion of how a single event might alter your life, as each person's experience is different and the essay is about your thoughts on the matter--not mine. Having said that, I can offer some ideas for you to take into consideration.
In "The Painted Door," John's wife, Ann, chose, to marry him, despite the fact that he is, in his own view, "dull-witted." The narrator tells us that he is "a slow, unambitious man, content with his farm and cattle, [and] naively proud of Ann." He seems homely, with his beard growth and weathered hands.
By contrast, Ann is clearly bored, restless, and lacking something to make her feel complete. When John leaves for one day, she busies herself painting the cabinets and door just to keep her hands busy, to keep her mind focused. She never sees anyone else, so John suggests Steven come over to play cards, because "you haven't seen anybody but me for the last two weeks."
John is a simple man, a doting husband, but Ann wants and needs something more--yet feels guilty for feeling that way. Thus, the first event that significantly altered her life, although it happens before the story, is marrying a man poorly suited for her.
The more obvious event that changes her life is her decision to sleep with Steven when they both become convinced that John will not make it home that night through the storm. This arrangement doesn't affect Steven, so far as we are told; there's no passion, no guilt (we aren't told how he reacts when they find John the next morning). Ann, however, sees the paint on his hand and knows the pain she brought him, and why he walked back into the storm, and we have reason to believe that this single discovery is a significant event in her life which will alter her forever.
My opinion is that life is made up of such moments upon which fate turns.