The speaker begins by calling themselves a simple person, then immediately hedges: "or, more so." That small stumble sets the tone. They like to think they're simple, but they aren't sure, and the person they're addressing seems far more complicated. Much of the poem is the speaker watching closely: studying this person's movements, expressions and words, trying to work out the reasons behind every choice.
The effort doesn't pay off. Even though the speaker is called "smart" and "kind," they admit they'll never understand more than a fraction of this other mind. The mood is a mix of admiration, frustration and quiet humility. The ending turns the opening around. "I'm a simple person" becomes "You make me a simple person," which can read as a confession that the other person's depth makes the speaker feel small, or perhaps that being around them strips everything down to one simple feeling: fascination.