The speaker is remembering a close friend called Mike, who seems to have mattered more than a simple friendship might suggest. At first the poem slips into "you," as if the speaker is describing a type of person many people might know: someone kind, protective, and respectful, who never pushed you into anything. He defended you so often that others assumed you were a couple, which even made his girlfriend jealous. Then the poem turns personal with "my Mike is gone now," and we understand the speaker has been talking about herself or himself all along.
The mood is tender and wistful, with a little ache running underneath. The speaker admits Mike could be flirtatious and annoying, but that doesn't weaken the love. The moon-and-stars comparison captures the longing neatly: the moon waits in an empty sky for company. The poem ends in plain thanks, saying Mike will always be a part of the speaker.