"A lot of guys already come to say goodbye because they're not doing the Classics," Boonen said ahead of his time trial ride on the final day of Tirreno-Adriatico.
"Many guys came to me and said, 'We are thing thinking about the races we're doing and we won't have that many races together any more, so I've come to say goodbye and good luck for the rest of your life.'"
The 36-year-old won the Tour of Flanders three times and Paris-Roubaix four times.
He will race Milan-San Remo, Ghent-Wevelgem, the Tour of Flanders and Paris-Roubaix before calling it quits. He will retire with Paris-Roubaix, where he has the chance to set the record of five wins.
"No, Quintana, no! Hahah!" Boonen said when asked if the Tirreno-Adriatico winner said goodbye in the last week.
"But it's strange, someone comes to say goodbye and you realise that there are not that many races left any more.
"I know how it was when someone else used to stop and you'd go up to him and tell him how strange it'll be when he's not there any more. Then he's not there any more and nothing changes! The race goes on.
"With Johan [Museeuw] it was really strange because he was in the team, with Peter Van Petegem and all those guys. You have a good relationship with them and then suddenly they are going away from our own protective world and out to the big world."
Bradley Wiggins also closed a chapter with Paris-Roubaix. He stopped racing with Team Sky in the Roubaix velodrome in 2015 and began to focus on the Hour Record and Olympics.
"When was the last time I saw him? In the Tour of California, yeah," Boonen added. "Then Roubaix? Yeah, but he didn't stop after Roubaix! He stopped already 15 times! It doesn't work that way, eh! Or you stop or you don't stop and you keep racing!"
Boonen began his season with the Tour de San Juan in Argentina and raced the Tour of Oman before debuting in Europe this season.
"I'm not thinking about all the last things I'm doing, I'm thinking about that are coming up. I'm not aware that I'm doing things for the last time, otherwise, I don't think I'm focusing in the right way," continued Boonen.
"That's also why the last few months weren't hard for me, it was easy, I was in Argentina, Oman, I was training camps, the entire world was turning, and I just did my own thing like I did every year.
"In Belgium, it gets more busy. Of course, now we have two press conferences coming up for Flanders and Roubaix, it'll be a little bit more busy than normal. Personally, though, it'll be strange."
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