I just stumbled across John Piper’s answer to this question, and it’s predictably excellent. Some of the points he makes:

• The intellectual appeal of the system of Calvinism draws a certain kind of person, who doesn’t tend to be the warm fuzzy type.
• When people come to believe in the doctrines of grace, they’re often amazed that they missed them for so long, and sometimes angry that their churches haven’t been teaching them.
• Calvinists do want to convince others of the truth of these doctrines. At best this is not out of elitism, but it sometimes comes across that way because of our sin.

Piper does a good job of acknowledging that this stereotype exists, admitting that it’s often accurate, and explaining why it is often inaccurate. It’s a much better answer than a cliche like “A proud Calvinist is an oxymoron.”