Two critical decisions shape long-term happiness: choice of spouse and choice of profession. Evolutionary psychologist Gad Saad explains why 'birds of a feather' compatibility matters more than opposites, how creative work drives purpose, and why birth order influences innovation and openness to experience. The two decisions with greatest impact on happiness are marrying the right person and choosing a fulfilling profession; both require strategic choices rather than luck. Shared fundamental values and life principles matter far more than identical interests; compatible partners aligned on core beliefs sustain long-term satisfaction better than complementary opposites.