Once in a while we need to stop and check out motives. After time, is our heart still in our work, or are we just going through the motions? That is just as faulty as if we are boasting about our deeds. Even good deeds and good intentions can lead to sin if we lose focus of why we are doing them.
The devil doesn't always attack us head on, often he skews the good we are tying to do to be ineffective and manifest sin. Pride is a big one in this subject. We want to share with the world what we do and forget to give credit to He who makes it possible. God provides the funds, the time, and the opportunity. Without God we would be nothing - we would not even exist. All the glory should go to God, keeping none for ourselves.
Even just going through the motions leads to pride because we can do it, so we should. This exerts a sense of duty, but not love. The Lord wants us to love others because He loved us first, to help others because He helped us first. When we lose sight of God in our work, it is just that - work. It becomes ineffective and a place for sin and resentment to breed.