"No one else has access to the world you carry around within yourself; you are its custodian and entrance. No one else can see the world the way you see it. No one else can feel your life the way you feel it. Thus it is impossible to ever compare two people because each stands on such different ground. When you compare yourself to others, you are inviting envy into your consciousness; it can be a dangerous and destructive guest."
Explanation
O’Donohue is basically saying that you live inside a world no one else can enter. Your memories, moods, fears, hopes, the way you feel the weather or hear a song, no one experiences those things exactly the way you do. That means comparing yourself to others is like comparing two completely different planets and then blaming yours for not having the same sky.
Think about scrolling through social media, seeing someone’s “perfect” life, and suddenly feeling less than. You’re judging your rich, complicated inner world against their carefully edited surface. This quote is an invitation to step back, remember that your perspective is utterly unique, and treat it with a bit more respect and kindness instead of letting envy move in and rearrange the furniture.