We self-centered humans are not naturally thankful. A young children we yell "Mine!" "Mine!" We think that we deserve everything, Everything we see is ours
Thankfulness is a habit that we can learn. By becoming aware of the special services that people do for us, (hopefully) we learn that we don't deserve anything
Benefits of being thankful.
When we enthusiatically express our appreciation to those who do stuff for us, or who give us things, they become more glad that they did something. Conversely, if they see us grumbling (even silently)about something that gave us or did for us, then it negatively affects they spirit. They often become less willing to something for us again. If it is in their power, they may even take back that which they gave us.
Thankfulness is a great gift in and of itself. It spreads cheer wherever it goes.