When you are a professional athlete(especially one who was as good as Melo) someone saying anyone is better than you makes you feel slighted enough even if it is Lebron,not to mention everyone has been making a joke out of him all off season. Its not that hard to believe he would respond.
The guy who said Lebron is the best ever player by a bit of a margin offered Melo the vet minimum. The commenter has a point, though it was made in a very insulting way.