Oskar may feel the need to tell people how he feels about them or just to simply say something to cheer someone up. In the Raisons D'étre part of the wiki, someone said that he finds "reasons for being" because he feels that he is insignificant. Insignificance of people
"'You're incredibly beautiful,' I told her, because she was fat, so I thought it would be an especially nice compliment, and also make her like me again, even though I was sexist."(44)
"I told her, "I'm sure people tell you this constantly, but if you looked up 'incredibly beautiful' in the dictionary, there would be a picture pf you."" (91)
Oskar's life is filled with women, his mother and his grandmother. He may feel compelled to tell everyone that they are beautiful just because the people in his life are so wonderful and beautiful to him.