मनःप्रीतिकरः स्वर्गः नरकस्तिद्विपर्यायः ।
नरकस्वर्गसंज्ञे वै पापपुण्ये द्विजोत्तम ॥
What gives you happiness is heaven. The opposite is hell. When you do good deeds, your conscience makes you happy. When you hurt some one, it makes you sad. Hence the Heaven and Hell are also associated with doing good deeds and bad deeds respectively.
- Vishnu Purana