KGBlack talks to nostalgia as if it were a person, someone who turns up without warning. The speaker calls it a "weird feeling" and compares it to a happy child: playful, unpredictable, impossible to schedule. A photograph is one of its favorite tricks, but the speaker admits that pictures work a bit like dreams, so some details slip away. The memory comes back, but not completely.
The speaker's feelings are mixed. Nostalgia is "sickening" and also "sweet yet bitter," so remembering is a pleasure that hurts a little. The last stanza softens the mood. Nostalgia says "Goodbye for now" and promises to return, and the speaker calls it a "childhood friend." That ending feels affectionate and a little resigned, as though the speaker has accepted that this visitor will keep coming back. The overall mood is wistful and gentle, with a hint of ache underneath.