*I* wanted to scream at him that coupling isn't the only path in life. At the very least, his focus on it is probably a part of his problem. (The old stop looking and you'll find someone...or at least you'll make peace with not finding them.)
Although the trigger for his meltdown isn't so much being single as his friends marry, it's the change to his lifestyle that comes from the changes in his friends' lives (aka growing up). They're not four single kids hanging out any more. In Significant Other 2, his married friends will all have toddlers and he'll really go out of his mind.