Grandma says this to Oskar because she hopes he does not experience loss.
When Grandma says that she hopes Oskar does not ever love anything as much as she loves him, she is not saying that she hopes he does not love. She hopes that he never hurts. She understands that with intense love often comes intense loss. When you love something too much, you begin to fear losing it.
Even as young as Oskar is, he fears losing those he loves.
We need enormous pockets, pockets big enough for our families, and our friends, and even the people who aren't on our lists, people we've never met but still want to protect. (ch 3, p. 73-4)
In the aftermath of 9/11, loss was being experienced everywhere. Oskar had already lost so much. Grandma is just hoping he never has to lose as much again. She knows how much losing his father has affected him, and she knows that he loved his father. She also realizes that he feels alone, and disconnected. He is filled with fear, not just for himself but for those all around him. She wishes for him a future without that fear or loss.