The purpose of gratitude is to help with the realization that we are not alone and that we have everything we need. God does not need us to thank him for anything, God gives with an open heart free of charge and asks nothing in return. Gratitude is not for God but for the individual.
An individual who practices unrelenting constant gratitude will always be at peace for they will always know that they have everything that they need at any given time. That "but" that you just heard in your mind is you trying to convince yourself that you do not in fact have everything you need.
What does one need in this life? To be alive, that's it. Otherwise, you would be dead in which case you wouldn't need anything. Your here to serve a purpose, there is a reason for everything and everyone, if there wasn't a purpose, nothing would be here. Since we are all here for a reason we cannot die until we fulfill that purpose. We are essentially invincible. The kicker is most of us don't get to know our purpose, so we never know when we are going to die. That would take the fun out of it. If you know what your purpose is, you might decide to "do it early" or "put off doing it" as to live longer. Sorry but there is no cheating fate. We all have a different and individual purpose for being here and then we die.
Until we fulfill our purpose and die our only "need" is to be alive in order to fulfill our purpose. As already explained it is an impossibility to die before we serve our purpose. The fact is we don't even "need" to be alive. One could say that we don't "need" anything. If you're reading this that means that you are in fact alive and that means that you already have way more than you "need". You have had enough food and water to still be alive your entire life. You have gotten to see beauty all around you every day without even being aware of it. You have gotten to hear music which could be argued serves no purpose other than for the pure joy of it. Music simply exists for the sake of existing.
Life could be contemplated in the same way music can be. Existing simply for the sake of existing. Is it possible that your purpose is to have fun, enjoy the beauty of it all, and appreciate the peculiarity of it all? Maybe you won't die until you've had all the fun and seen all the beauty that you can handle. Or maybe there is a deeper purpose to it all. Humans have been questioning the purpose of it all since we first became self-aware. Whether we are here for no reason whatsoever or for a very specific reason, it doesn't change the fact that we are here, and we will be here until it is our time to die. So, until that time there is nothing that we need and yet we have everything and then some.
Anything you think you "need" is only you telling yourself that thing you desire will make you happier. You only think you "need" something else in order to be happy because you are not grateful for what you have, simple as that. Be grateful for everything you have, and you will see there is nothing else you "need". Love and Peace!