the pursuit of happiness is a force of nature harnessed by evolution

The existence of both happiness and suffering creates a well-being gradient/field which permeates the multiverse. Happiness is an attractor state which exerts a pull on minds analogously to how gravity exerts a pull on matter. Increasingly complex and sentient minds evolved to harness this force more aggressively.  

The capacity and tendency to suffer is specifically built into the structure of mindbrains. For example, a mindbrain constructed such that sex feels delightful while a lack of sex feels miserable will be pulled towards creative solutions to get sex, and thus advance the interests of the genes who designed it.