Being "thankful for what you have" is touted as the thing to do when you wish you had more, when your parents didn't know what else to say, or when church leaders wanted to keep their members from losing faith. However, this post is NOT about those things. Yes, it's about gratitude and appreciation, both emotions that fall on the positive side of a scale of emotions that run the gamut from bad to good. Yes, it does mean being thankful. But, no, not because "you should" or "God will approve" or "others have it worse". Instead, let's look at it from a different perspective.
Have you ever been pregnant, and all of a sudden, you notice pregnant people everywhere? Or, have you ever started looking at buying a certain car, and you start to see that car all the time on the roads? Or have you noticed a certain breed of dog you have never seen before, and you start to seeing that breed of dog all over? It's called the Baader-Meinhof phenomenon, which names frequency illusion. In reality, the number of pregnant people (or that certain car or breed of dog) has not changed at all. You now just notice it more. Let's apply this to gratitude and appreciation (or any emotion), as well as complaining (or any behavior).
First though, let's consider another idea. In cognitive-behavioral therapy (CBT), there is something called a CBT triangle. Here is a visual.
The triangle gives us a visual reminder that what we think affects how we feel and act, how we feel affects how we act and think, and how we act affects how we feel and think. Thinking, feeling, and doing, are all connected. If you think about something you are really thankful for and do this every day, you will start feeling some sort of way. You may feel appreciation for those things. You may feel gratitude for those people. In fact, you might be compelled to do something like tell that person how much you love them. Conversely, if you complain everyday, you may feel like your life is terrible. You may feel like there is no hope. You may act irritable and rude.
Now, let's combine both principles: Focusing on something to notice it more, and thinking a certain thought to affect how you feel and behave. Now, both principles are actually in effect whether or not we are intentional about what we are noticing or thinking. Here is an everyday example: You tell yourself that your drive to work sucks and people don't know how to drive. You notice a driver going too fast, cutting people off, and that reinforces to you that your drive to work sucks. You start noticing more bad drivers. When you get to work and talk about your terrible drive to work, more people who hated their drive to work reinforce how terrible driver's are in your town. What happened? Noticing bad drivers, by law of Baader-Meinhof phenomenon, caused you to notice more bad drivers. Thinking how terrible your drive to work is, caused you to talk about it at work to others who can relate, reinforcing your thought. And, very likely, after all that, you feel some kind of way - probably on the negative half of the emotional scale, like miserable or unlucky.
Finally, let's apply this to gratitude and appreciation. Let's say every morning as you drink your tea or coffee or get ready for the day, you think about something you are thankful for. Today you think about how wonderful it is to have air conditioning on hot summer days. You start thinking about and noticing every place you go there is air conditioning on hot summer days. You feel some kind of way - mostly likely on the positive side of the emotional scale, like how lucky you are to have access to air conditioning. You are quite possibly in a state of appreciation.
In summation, even though nothing in the world is functioning any differently, if you start focusing on and noticing things you like, appreciate, or are thankful for, you, by the laws stated above, will notice more to be grateful for and will feel more positive (and will start doing/saying more positive things). The negative things you used to focus on are still there, but you notice them less, and you start to feel better, even though they exist.
Of course, the converse is true as well, but given the choice, and you are given the choice, which would you chose? I would choose gratitude and appreciation.