The speaker is worried about a long silence between himself and someone he cares about. If they never speak or meet again, that person won't see how age has marked his face, or learn that he was always in their thoughts. He might even die without their knowing.
Then the other voice answers, coldly: everyone dies, everyone gets old, and why should it matter whether he thinks of them at all? The speaker takes this in and slides into a kind of wordplay about "nothing." Does having nothing matter, or does "no thing" matter? He decides that what matters is the vacuum itself. If all anyone really cares about is themselves, then when you disappear, the world does too.
The mood starts out tender and a little sad, then turns bitter and numb. The repeated "nada" at the end sounds like a shrug, as if the speaker is protecting himself from being hurt by deciding that nothing counts.