how can we gain happiness?

Happiness depends on living in accordance with appropriate virtues. Virtue is a disposition rather than an activity. That is, a virtuous person is naturally disposed to behave in the right ways and for the right reasons, and to feel pleasure in behaving rightly. Virtue is a mean state between the extremes of excess and deficiency. This mean varies from person to person, so there are no hard and fast rules as to how best to avoid vice.