If Aragorn wasn't courting Arwen, he would have ignored the letter. After all, he was chieftain of the Dunedains; he could always make up some excuse and say he must stay with HIS people. But as the Man was seeking Arwen's hand, among other things, he needed to be on good terms with his foster father. It was exasperating how Elrond exploited this weakness. The Elf- Lord was hinting in not so subtle terms that if Aragorn did not return at once, he could forget ever seeing the Lady again. She would be sent to Lothlorien before he could say "Ni", which gave the Man no choice but to track grudgingly back to Rivendell.