In other words, don't just be grateful. Express your gratitude, say 'thank you' and mean it.
If I might share a word of caution, please don't be indiscriminate in your thankfulness for that takes meaning away from it. I once had a supervisor who would say to me, "Thanks a ton," about everything I did. I fully believe that he thought he was showing appreciation, but in my mind, there was no acknowledgement of special effort for I received the same level of praise even when I performed below my expectations.
I doubt that any of us work for praise. However, the right amount of praise, at the right time, sure goes a long way. It is the optimistic thing to do. Share your appreciation and soon you'll realize that you are sharing optimism too.
