Social Environment Impacts the Reward Pathway
Just as food gives us a jolt of pleasure by firing up the brain's reward pathway, so do our social interactions and environmental circumstances. If you have a great job, pleasant relationships, and are respected by your co-workers, you're going to feel prettygood. On the other hand, if you're bored with your job, have troubled relationships, and aren't respected by your co-workers, your reward pathway isn't getting much stimulation.
Individuals who don't receive enough natural rewards from their social environments are more likely to stimulate their neglected reward pathways with drugs.
Morgan, D. et al (2002). Social dominance in monkeys: dopamine D2 receptors and cocaine self-administration. Nature Neuroscience 5, 169-174. doi: 10.1038/nn798