When someone brings a gift to our door, we receive it and say thank you. We have not opened the gift as yet, but the thank you is an acknowledgement of the gift.
It's the same with prayer. When we pray we must not forget to thank God as an acknowledgement of the gift not yet opened.
Thank you for a great reminder! One of my favorite prayers, that spice up my level of graditude is..."God if I can't have what I want, thank you... and thank you for giving me what I have".