The reason a Christian can give thanks is because of the grace of contentment. Contentment is in our hearts when we give thanks. When our hearts are content, we're obeying to the 10th commandment, “You shall not covet.”
There seems to be a good deal of misunderstanding about the relationship between personality type and the Christian life. J.C. Ryle said, “Grace does not alter natural temperaments, when it changes hearts.” There is something in every personality that reflects the image of God and shines brightly from a holy heart, and there is something in every personality that has its own unique temptations to sin because of the brokenness of human nature.