We become what we think about

“We become what we think about”, said Napoleon Hill (author of "Think and Grow Rich")

Thinking ungrateful thoughts and taking things and people around you for granted, will bring you more things to be ungrateful for. And you will become, or continue to be, an ungrateful person.

This is easily reversed by starting a Gratitude Diary. For people who complain a lot, finding things to be grateful for will be difficult at first. A little perseverance over a month or so, for 5 minutes each day will turn that attitude around, and miracles will start to occur in life.