I don't believe that it is just for God's sake that He tells us over and over again to be thankful. It is so incredibly healing for us! Even if it did nothing for us, He deserves all the thanks we can give but I think when we start obeying Him and begin praying with thanksgiving the way He tells us to we will be blessed far beyond what we can imagine.
God gives us hundreds (thousands? millions?) of gifts every day and many of them we don't even notice. Instead we are busy thinking about our struggles, the hard things we face, the loneliness in our lives. When we start looking for those gifts instead and writing them down it fills you with such a great sense of overwhelming love - realizing just how much God cares for you. Taking the time to write them down can retrain your thinking. Romans 12:2 tells us to be transformed by the renewing of your mind" and I think obeying God in giving thanks is part of the way to do that.
Go beyond just being thankful for family, friends, a home, a job, enough to eat and a vehicle to drive. Start thinking about all the people in your life and then think about what about them makes you thankful for them. Start noticing the beautiful things God puts in your life whether it be a mushroom you chance upon out walking, a frozen puddle that has pretty ice designs, the bark on a tree. Start noticing and get specific in your thankfulness. Give thanks for specific interactions/conversations you are able to have. Notice the sounds around you and give thanks for the ears that can hear and the beauty of music, nature, breathing, etc. Think through the things in your life that have molded you into who you are today. Give thanks - even for those hard things. Once you get started writing down what you are thankful for and truly noticing things you will find it is pretty hard to stop.
I would love to hear if you practice this already and if you do what affect it has had on you. If you don't do it would you please, please try? I know you will be blessed.