Why is suffering the second characteristic? People in general think that there are also many elements of happiness in life and do not sense that samsara only has miseries. The reason why we so actively and enthusiastically engage ourselves in the pursuit of wealth and fame of this world is because we believe there is happiness to be had in these worldly achievements. This view comes from our belief  that life in the god and human realm is basically a happy one. The Buddha requested that we regard all phenomena in samsara as suffering, which not only is a request from the Buddha but also a fact. Conversely, if the Buddha did not tell the truth, we would not need to comply either, whether or not the Buddha has requested. Although in real life we can see and experience suffering around us at any given time, we tend to easily forget what we have witnessed. As a result, the miseries we so witnessed cannot help us discern the true nature of samsara. This is why the Buddha taught us to regard samsara as nothing but suffering. 

~Depicted from From Believers to Boddhisattvas