It cannot be wholly about our relationship with God since folks who self identify as atheists or agnostics as well as socially religious folks volunteer for any number of altruistic endeavors from animal shelters to coaching inner-city baseball teams to soup kitchens.
It appears that there are two distinct reasons people volunteers: (1) To feel good about themselves; or (2) because they believe they have a responsibility to do so. Both can propel folks into doing good works and both have potential problems attached.
If I am only helping you to make me feel good it puts a LOT of pressure on you to
(a) be appreciative (If you have ever been expected to be grateful you know this can be a very crumby position to be in.
(b) be acquiescent (If you have ever been told what to like because someone expects you to like what they want
you give you, you know this is no fun either.)
(c) be less able to care for oneself (If you have ever needed help and been left feeling judged or inferior
because of it, you know how demoralizing this is.)
Neither is it much fun to be the recipient of begrudging help. It is kind of like the difference between have a pleasant time with everyone helping to clean up after dinner versus having one person who is sulky, negative and generally ill tempered banging every pot and pan.
Another challenge is to keep up the energy needed to continue helping if the recipients of our help are too broken themselves to both bear their burdens and boost our egos. And begrudging as we go along trying to keep a happy smile pasted on while we are helping against our true desires does not endure, especially if circumstances change and the giving becomes more burdensome.
That is why things are different for me since I started doing things out of gratitude for all I have been given, all I have been forgiven, all the joy in my heart because God loves me. I'm not saying it is easy to be merciful to folks who are challenging or angry or very, very, very much in need. But when I see God's face in their's it is much, much, much easier. God may, and often does, ask me to help where no return is possible either in gratitude or humility, and neither will I get much "credit" in the world. And I will still feel burnt out and put upon sometimes, but God can get me powered back up and perspective adjusted with increasing speed.
So, if you are feeling burnt out, under appreciated, and dragging yourself along, have a chat with God about it. He may have a better approach, a drink of refreshing water, a healing for the pain your soul bears because of the sadness you have seen others bear. God can help you get out of your own way so you can truly give love to the brokenhearted without loosing yourself.