The speaker explains why they talk to themselves. When people ask, the answer is simple: they have imaginary friends. Most people react by giving them a strange look and walking off, and nobody stops to wonder why someone would need friends who aren't real.
The poem answers that question itself. The speaker says real people can be mean and don't value them for who they are. Imaginary friends are the opposite: they don't judge, don't tease, and are kind. The plain, repeated lines, especially "That is why I have imaginary friends," make the speaker's reasoning sound steady and sincere, almost like someone saying it over and over to be understood.
The mood is lonely and quietly sad, though not angry. The last two lines are the most striking. "Real friends, / Are mean" suggests the speaker has been hurt enough to stop trusting that real friendship can be kind. The poem leaves us with a feeling of isolation, and a wish to be accepted.