Thankful people enjoy life more. Thankful people are fun to help and do nice things for. Thankfulness is one of the primary marks of humility. We live in a proud, rights-oriented world that spends too much energy whining and complaining about what we deserve and not nearly enough energy being thankful for all the good things we have that we don't deserve. (Let's also be thankful for all the bad things we don't have that we do deserve.)
1 Cor. 10:10