He wanted to retire and live the rest of his days with Benjamin. He didn't get to do those things even after the age when he was allowed to. Napoleon would not let any of the animals retire at the age they were supposed to and Boxer just went along with it because, he thought it was the right thing to do. And when Boxer got injured while working, he didn't want to let Napoleon down so he kept pushing himself through the pain and agony “he said, he had only one real ambition left and that was to see the windmill well under way before his retirement”(9.111). He ended up collapsing in the middle of a work day. Napoleon himself showed sympathy towards him and said he would take him to the doctor, but when the people who picked up Boxer started to drive away Benjamin noticed they were going to turn him into glue. Napoleon denied all claims and said it was a mistake while also stating the following “The van had previously been the property of the knacker, and had been bought by the veterinary surgeon, who had not yet painted the old name out”, (9.125). But we all know that is not true because it's way too far