The speaker is talking to someone who has pulled away. Each line lists a small hurt: no calls, no texts, no time. After every one, the speaker answers with the same calm phrase, "I understand." It sounds patient and mature, like someone trying hard not to make a fuss.
Then the last line changes everything. The speaker says they hope the other person will understand when they're seen with someone else. The phrase that sounded like acceptance now works as a warning, or a quiet bit of payback. The patience was real, but it had a limit.
The mood is hurt, controlled, and a little bitter. Nothing is shouted. The anger comes through in how neatly the pattern turns on the reader. The poem also suggests the speaker has been understanding for a long time, and that being understanding has started to cost too much. Moving on is presented as a fair response, not a threat.