The only thing I can truly know is that I exist, nothing else. Since that is fundamentally the only truth I can grasp, then how do I know that you're real as well? What does it mean to be real?
But if I take this thought to its logical extreme, then only I can be real, at least according to myself. Then, are you all fabrications of my own mind? Aspects of myself?
If only I exist, then am I God?