Drive is a single sentence, and everything in it hangs on the word "if." The speaker says they would drive forever, but only on one condition: that it brings them back to someone they miss. The road isn't the point. The person at the end of it is.
What the speaker feels is longing, plain and unguarded. There's no explanation of who "you" is or why they're gone, so we're left to guess. The poem doesn't say whether the person has died, left, or is simply far away. That gap makes the feeling hard to pin down, and easy to recognize from our own lives.
The mood is quiet and a little hopeless. "Forever" suggests a trip with no end, and the wording hints that the speaker doubts the drive would actually work. It reads like a thought that comes at night, behind the wheel, when missing someone feels bigger than any distance.