The speaker describes a hidden part of their heart, shut away so deeply that it had no key at all. It was meant to be impossible to enter. Then another person, singled out in the capitalized "YOU," got in anyway. The shift from the quiet, flat tone of the opening lines to that sudden shout makes the break-in feel abrupt, almost like a shock.
What unsettles the speaker most is not just being seen, but being understood better than they understand themselves. The last line, "that my friend terrifies me," sounds casual and a little blunt, yet the fear is plain. The mood is tense and vulnerable. There's a hint of admiration for the person's insight, but the dominant feeling is unease. Having no control over who gets in, and finding out someone knows your inner self better than you do, can be thrilling and frightening at once. The poem leaves that mix unresolved.