I find this so true.
There will always people with less than you and there always people with more than you. Any one of these people may be happy or they may be sad.
So don't hinge your happiness on what you have. Appreciate and be thankful for all that you do have in your life.
And if you need a reminder that happiness comes from within and not from the things surrounding you then remember that someone else is happy with less than what you have.