Two short lines, and almost everything in them is about helplessness. The speaker is telling someone, or perhaps just telling themselves, that this person is leaving. The second line admits the hard part: there is nothing they can do to change it.
What stands out is how little the speaker says. There's no pleading, no anger, no list of reasons to stay. The bare wording makes the moment feel quiet and stunned, as if the speaker is still absorbing what is happening. The missing words might suggest that anything more would be pointless, or that the feeling is too big to put into language.
The mood is sad but restrained. The line break after "leaving" leaves a small pause before the admission, and that pause is where the ache sits. We don't learn who is leaving or why, so readers can bring their own situation to it, whether that's a breakup, a friendship ending, or a loss of some other kind.