Holden used Allie's glove as a reminder of Allie himself. Allie had been one of the few people that Holden was close to. The only person he had showed the glove to was Jane Gallegar. Holden still remembers the exact date of Allie's death. In response to Allie's death, Holden broke all the windows in his garage. Even though Holden doesn't talk to anyone about Allie, we can see that he is really having a difficult time dealing with his death. Looking at the baseball mitt simply intensifies Holden's feelings of isolation because he really has no one to talk to about his emotions. Everybody else in Holden's life seems to be moving forward, and Holden is stuck, as his name suggests, "holding" on to his memories and sense of loss surrounding the death of his younger brother.
Holden alone understands that Allie's baseball mitt is a tangible reminder of who Allie was and what he valued. The fact that the mitt has poems written all over it in green ink so that Allie could entertain himself during slow times in the outfield is a poignant reminder of his personality. Because Holden keeps the mitt to himself for the most part, it isolates him in his grief. Anyone to whom he shows the mitt would not recognize the deep meaning Holden attaches to it; they wouldn't truly appreciate this quirky expression of Allie's charm. Holden seems to fear that, over time, Allie will disappear in people's minds, and that it will be as if he never existed. Holden hangs onto this palpable reminder of his brother, but in doing so, he closes himself off to others who might be able to understand and appreciate his loss.
Allie's glove symbolizes love and empathy in the novel. Holden has trouble sympathizing with others and fitting in. Allie was the one person Holden could truly identify with and care for. When Allie died, Holden punched out the windows until his hand turned bloody. He can't even make a proper first because of that. Holden is still very much grieving for Allie and carries Allie's glove around as a sentimental reminder. The fact Holden needs to carry around the glove to feel better is a sign that he is isolated from the rest of society.