Opportunism is presented to each one of us every single day. Certainly, not all opportunism is negative. If you are working for a company and the job of your dreams opens up in a neighboring department and you are the first one to hear about it, then the benefits for you are great and no unethical behavior was exercised. There is a debate about whether great people are born or put into situations that make them great. If great people are formed by situations, it is this opportunistic behavior that they exercised that makes them great.
There are also times as this post discusses that people do not take advantage of a situation for various reasons. I will present a couple simple situations and the different reasons for why opportunism was not exercised. One example is my friends and I were taking a cab back. My friend paid and the cab driver gave him too much money back, which my friend then returned the excess money to the driver. This is an example of my friend being a "good citizen". He acted honestly most likely because of his moral values of not wanting to cheat this man out of the money he deserves. A different example might be a man giving up his seat to a beautiful woman. Opportunistically, it would be better for the man to sit down, yet he yields the seat. Such action might be viewed as one being a good citizen and courteous. Another very plausible explanation for this behavior might be that the man seeks personal gain from his kindness, like a phone number of the strikingly beautiful woman. Another reason why people might not partake in opportunistic behavior because they fear the consequences of using the opportunity. For example, stealing when no one is looking. The opportunity is there, but a person might be afraid of the punishment to come from being caught.
I believe that there are very different reasons for people to not exercise opportunistic behavior. It might be because of their morals, their potential gain from not partaking in the behavior or their fear of punishment from the exercise of the opportunity.