Constant happiness is rare for the human body because its [happiness and unhappiness] are in accordance with the maturation of pious and impious karmic reactions.

NOTE. In The Nectar of Devotion (Chapter 1), Śrīla Prabhupāda explains how sufferings are due to the maturation of our impious karmic reactions:

Sinful activities are of two kinds: those that are mature and those that are not mature. The sinful activities for which we are suffering at the present moment are called mature. The many sinful activities stored within us for which we have not yet suffered are considered immature. For example, a man may have committed criminal acts but not yet been arrested for them. Now, as soon as he is detected, arrest is awaiting him. Similarly, for some of our sinful activities we are awaiting distresses in the future, and for others, which are mature, we are suffering at the present moment.

In this way there is a chain of sinful activities and their concomitant distresses, and the conditioned soul is suffering life after life due to these sins. He is suffering in the present life the results of sinful activities from his past life, and he is meanwhile creating further sufferings for his future life. Mature sinful activities are exhibited if one is suffering from some chronic disease, if one is suffering from some legal implication, if one is born in a low and degraded family or if one is uneducated or very ugly.

There are many results of past sinful activities for which we are suffering at the present moment, and we may be suffering in the future due to our present sinful activities.