Allegri will finish up at the San Siro at the end of the season, but the 46-year-old publicly made his intentions clear just prior to the New Year - after Milan's struggles in the Italian top flight saw them sitting mid-table.
The Italian said he informed Milan chief Adriano Galliani prior to Christmas of his desire to resign come mid-2014.
"I was very clear, this was a decision I took last summer that I have communicated to Galliani before Christmas," Allegri said following their 3-0 home win over Atalanta on Monday.
"It is not something new for me considering the rumours about the new coach from next season.
"In Italy sometimes all these changes can create clamour amongst people.
"These decisions are normal business and Italy will have to get used to it.
"We are all professionals, both players and coaches, and we must perform well for the whole length of our contract."
Allegri said his stay in the top job in Milan always had an expiry date, with the club languishing in 11th spot through 18 matches.
"It is not possible to stay in a club forever," he said.
"I have been in AC Milan for four years, I have done a good job and I am very happy for that.
"I need to thank the vice-president, Adriano Galliani, who both provided me a good squad that could achieve the objectives agreed before the start of every season.
"So far we met all these expectations; the only thing we are far behind at the moment is the third place. However we are competing for the Italian Cup and the Champions League; I am very confident as long as we keep working hard and within tranquillity."
Brazilian Kaka said it felt 'beautiful' to net his 100th and 101st goals for Milan, during their thrashing of 15th-placed Atalanta.
The 31-year-old struck either side of half-time at the San Siro, before setting up Bryan Cristante for the third - the teenager's first goal for the club.
Kaka held up a shirt with '100' on the back after his first goal in the 35th minute, and conceded post-match it was a milestone he never thought he would reach.
"I can't explain the emotions I feel for these 101 goals with Milan," Kaka said.
"If I think about it, it's a lot of goals in seven years here. It's beautiful, very beautiful and then the comparison with Gianni Rivera (122 goals for Milan), one of the greatest in the history of Milan, therefore I am very happy to be part of history.
"When you are waiting to score such an important goal, it's normal to be anxious. Luckily, today I made it and I am very happy to start 2014 this way."
The 31-year-old said he always believed he could still match it with the best in the big European leagues, despite being sold off by Real Madrid last year.
"I never thought I was done as a player. I always thought I could do my job," the Brazil international said.
"I know I have been below the par I myself was expecting, but I never thought I was over and could give up.
"You can go to Madrid and ask Real if I ever gave up training or lost motivation.
"They will tell you at least I have been a true professional on that level. Then things can go wrong, but I never thought I was done and I always kept the faith football could still give me a lot of joy."
