Back in season three, the employees were all invited by Kelly to join her at a Diwali celebration. At the celebration, Michael was introduced to Hindu culture, and learned about arranged marriage and how quickly Kelly's parents got married. Michael then proceeded to propose to Carol in front of everybody, which caused an awkward moment for the two of them. As the evening went on, he acted even more like his dumb self, making Pam feel very uncomfortable, to the point where she would only give him a ride if he sat in the back.
Four and a half years later, it was time for Michael to make another proposal, albeit to a different woman. Although over the years Michael has learned to think and listen to what others have to say, his stupidity still found a way to override any precautions he has undertaken. As Michael was emptying the gasoline on the parking lot, Pam had to intervene as it was a disaster waiting to happen. In tricking Michael to sit down with her and some of the other men, Pam discovered just how much sacrifice Michael was willing to make for Holly in the cost of his ring. Pam convinced Michael to just keep things simple, while Jim opened up about his own fears when he proposed to Pam in the rain.
"No, I'm not going to be proposed to in the break room. That is not going to be our story. I should have burned this place down when I had a chance."
The message got through to Michael. Instead of all the other proposal ideas he talked about in the meeting room, he took Holly by the hand and gave her and the viewer a retrospect of her time at the Scranton branch. It was not the parking lot that was on fire, but a large amount of candles in the annex. As Michael started his proposal speech, the candles triggered the sprinkler, giving Michael that memorable proposal in the rain that he wanted.
"Holly Flax, marrying me will you be?"
"Your wife becoming be will I."
When the employees went into the annex to congratulate Michael and Holly, they also found out that Michael was going to be moving to Colorado. It is times like this that Michael's lack of thinking things through are not a testament to his stupidity, but rather, it is to his immediate willingness to make sacrifices for the people he cares about. His change of values since the break-up ensured that he would not be making the same mistake twice of not moving with Holly, and just goes to show how much Michael has matured over the years.