How can high levels of happiness backfire?

People who are very happy may also become complacent and not strive for a better life or better job.  The optimum level of well-being theory postulates that there is an optimal level of happiness where we are able to feel good about ourselves and our lives, but remain optimistic that there is room for improvement as well.  However, this is predominantly applicable to achievement tasks (such as work).  In interpersonal relationships, even very happy people do not seem to have a negative impact on their relationships, rather their happiness and their relationship quality seem to continue to increase. 

Thanks for taking the time to find the right answer. This is going to help with my final exam review.