Happiness is a state of mind. Some people are very happy, and others have varying degrees of happiness. We being only human have things we would like to do or have and those cravings, can lead to us not being happy until we have managed to fulfill them.
The saying 'grass is always greener on the other side' also reflects this human behaviour. Sometimes we assume things that are completely untrue. We assume that the person in the big house has what he wants and if we live in a room, how nice it would be to live in the big house. The person in the big house could be looking at the small house and saying that it would be nice to live in the small house. He may feel it is more intimate or easier to clean or whatever. We don't know, so don't assume.
How do we therefore control this. We must try to be a little more contented with our lot. It is perfectly OK to feel to that it would be nice to have a new car when yours is currently breaking down everyday. Life is full of choices and we have to prioritize those choices taking into account our circumstances. Is therefore a compromise as it is difficult to have everything we are looking for.
It is therefore important to appreciate what we have over the craving for what we don't, then this state of happiness can be reached with less not more.