Why we serve

I have so many thoughts in regards to the following passages:
Isaiah 58:6-10

⁶ "Is not this the kind of fasting I have chosen:
       to loose the chains of injustice
       and untie the cords of the yoke,
       to set the oppressed free
       and break every yoke?
 ⁷ Is it not to share your food with the hungry
       and to provide the poor wanderer with shelter—
       when you see the naked, to clothe him,
       and not to turn away from your own flesh and blood?
 ⁸ Then your light will break forth like the dawn,
       and your healing will quickly appear;
       then your righteousness ^[a] will go before you,
       and the glory of the LORD will be your rear guard.
 ⁹ Then you will call, and the LORD will answer;
       you will cry for help, and he will say: Here am I.
       "If you do away with the yoke of oppression,
       with the pointing finger and malicious talk,
 ¹⁰ and if you spend yourselves in behalf of the hungry
       and satisfy the needs of the oppressed,
       then your light will rise in the darkness,
       and your night will become like the noonday.


and the ever popular Matthew 25: 41-46

⁴¹"Then he will say to those on his left, 'Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels. ⁴²For I was hungry and you gave me nothing to eat, I was thirsty and you gave me nothing to drink, ⁴³I was a stranger and you did not invite me in, I needed clothes and you did not clothe me, I was sick and in prison and you did not look after me.'
 ⁴⁴"They also will answer, 'Lord, when did we see you hungry or thirsty or a stranger or needing clothes or sick or in prison, and did not help you?'
 ⁴⁵"He will reply, 'I tell you the truth, whatever you did not do for one of the least of these, you did not do for me.'
 ⁴⁶"Then they will go away to eternal punishment, but the righteous to eternal life."


There are many oppressed, hurting, poor, suffering, and hungry people out there.  1 out of every 6 people in the world goes hungry.  Unacceptable.  The bible specifically speaks to us about those who are less fortunate imploring those of us with more to give, to help, to serve.  My question is why?  For our own salvation such as the parable of Matthew 25 refers?  I thought that our salvation was from the grace of God and not tied to any of our actions.  


Just a random thought of mine.  Really I believe that as Christians we should do all we can to relieve the suffering of others and I live my life this way.  Our ministry is to make disciples for Christ and spread the good news of God's love.  To love others is to help them, especially if they are in need.  But like all things I need to have a clear understanding of why I do something.  


I serve because I want to spread God's love.  I know that there is nothing I can do to earn God's grace and that none of my actions can even come close to paying the debt for my salvation.  But the passage in Matthew makes me think twice about that.  I want to be a sheep and be saved.  I think we all do, but I don't serve the poor, the hungry, the sick, a stranger, or a prisoner because I want to be saved and I don't believe that serving others will earn our salvation.  So what do we make of the passage?  


Would love any insights.  I'm not saying don't serve the needy - I'm just asking what is the underlying purpose behind it, for isn't that the key?