The movie, somewhat, shed some light regarding the dichotomy. William Wilberforce, a white and rich merchant, sacrificed his health to focus all his energy to his cause. What benefit can he reap from the abolition of the slave trade? He can get the good feeling of selflessness in other forms. He could have stayed in his manor and continue living as an elite. Instead, he took the cause and fought it with his life. He faced too many hurdles, yet his spirit and passion were as fiery from day one. It took him more than a decade to forward this cause. Many were lost but too much were to be gained. This act freed thousands of Africans and freed the future generations from the pit of slavery.
At the end of the day, William felt good when the bill of abolition of slavery was passed. In exchange of this good feeling came a price of his deteriorating health. Maybe this will be my new definition of selflessness. You are selfless if you something good in return you give up something of value.