I'd challenge the assertion that people that lack purpose are agreeable, and would like to see the research that supports that claim. Practicing some disagreeableness here. My own anecdotal evidence suggests that I've met agreeable people with purpose and disagreeable people without purpose.

If you do want to know what science says about purpose check out the book titled Life on Purpose. Haven't found the section on agreeableness yet, but it's got some scientifically backed evidence that purpose is positively correlated with health. It's also a good resource if you are still trying to discover your purpose. Don't get discouraged, it takes time for everyone, and you're free to be yourself throughout the process.