1) Acquiring status / proving yourself (qualifying for a better romantic partner, etc). More applicable for men.
2) Servicing existing debt (student loans, mortgage, etc)
3) Providing for a family (somewhat similar to 1 - but for future goals)
4) Securing a legacy/reputation
Although the most important drive in work is seeing a path towards upwards mobility (i.e. if I work hard then my lot in life can improve - not in an absolute sense, but in a relative sense. Meaning that I will be better off than my neighbor. This ties in to status games and mate competition from (1)).