What makes us seem so separate from one another is perspective.
Since birth, I've always experienced this world from my perspective - in my body, through my senses, in my private mental space.
And you have always experienced the world from & through your perspective.
You never experience the world through my perspective, & I never experience it through yours.
We can have secrets from each other. We can have disagreements. We can experience the very same thing - a movie, an ice cream cone, a beach walk - in very different ways.
This distinctness & exclusivity between our perspectives, & the discrepancies between our shared experiences, reinforces our intuitive sense that I am I, you are you, and any notion of our shared "Oneness" is just wishful or deluded thinking.
That's the ground-up view. That's how it honestly seems to most of us most of the time.
Now let's reapproach the situation from a top-down view. Here's how I see it.
There is One intelligence.
That One intelligence generates within itself the experience of life. It generates the universe, all the galaxies, the stars, the planets, and all the beings thereon.
Each of these things is an aspect of One, an expression of One, within the mind of One.
You and I, too, are aspects/expressions of One.
As you & I, One surrenders into a state of deep delusion & forgetting. So deeply that One believes with all its heart that it's not One, it's just "me," & you over there are something different & separate. The perspective game, the name game, the identity game, the pronoun game - they're all tools One uses to help One stay in the self-delusion for as long as One wishes. This allows One to experience the richness, extremes, surprises, & other treasures of self-hood.
Eventually, One removes the hood of self, in a process we call enlightenment & awakening. One reawakens to the truth of what all has always been: One.
Until then, we're in various degrees of the separateness game.
A metaphor I like is ocean & waves. One is the ocean. Selves are waves. A wave is always nothing but ocean, no matter how elaborately or forcefully it surges & rises & crashes. It is an expression of the ocean, and the very ocean itself. A temporary formation that arises, endures, and passes back into the sea.
You and I are waves approaching the same shore. From my perspective, I look over & see you, that wave, over there. And I look down at myself and see this wave, over here.
What we both struggle to appreciate is the immense commonality - the oceanness - underlying our relatively brief experience as seemingly separate arisings. We struggle to realize that we're both made of the same ocean, so matter how separate our crashing waves seem, and that our common destiny is to return to our oceanic state.
This is no accident or tragedy. It's this way because the One intelligence we are wants it this way. The more we realize & embrace the fundamental truth of Oneness, the less confused we are about our nature, which leads to wiser action, clearer thinking, & less emotional suffering.