Every day, we do things to help others. These can be easy, little, throw-away efforts, or they can be investments of time that are substantial enough that we must consider the opportunity cost as an investment of the company's most valuable resource - our time. In both cases, we are engaging in a transaction. When we invest in others and they respond with gratitude, the transaction is complete and we move on to the next task feeling that balance.
Sometimes, for a variety of reasons, the person who benefits from our work does not respond. They do not express their appreciation for an email or a recognition shared, or a requested task, or an opportunity provided.
Sometimes people miss the opportunity to complete the transaction, and say "thank you". When that happens, the transaction is left hanging, and can become an imbalance between people.
I love business and self-help books. One of the best is the Bible, of course. The Bible is not just about winning in the next world, but about being better in this one. There's a story in the Bible (Luke 17:11-19) where Jesus heals ten lepers, and all of them run off to tell their friends and rejoin society. Jesus asked why only one of the ten came back to thank Him.
I try to always express my gratitude for even the smallest thing someone does for me. I feel better knowing that the transaction between me and another who has worked on my behalf is balanced. This is a practice that has good benefits in the long term (as the Bible's guidance does) and also pays off in this world. By being thankful, I am encouraging someone to be willing to help me again in the future. That present-time benefit helps me, and it makes the culture around us better.