Perhaps utopia is a boring place. Or humans are curious and have a desire to explore and expand their knowledge base. For instance, what would colonizing Mars really accomplish? Mars is not suited for human habitation, much less the travel through the vacuum of space. McCoy has a great point on this in Trek 09. So, what's the point? It's not finding a better life-arguably, life on Mars would be more difficult. So, what's the driving force?
I maintain it is the human desire to explore and to satisfy that natural curiosity. And, I think Star Trek is the perfect vehicle for such an exploration.