It's actually pretty simple, but easier said than done.
The amount of money you have or make should have NO bearing whatsoever on your happiness, assuming you have enough to cover basic necessities like food and shelter.
If it does, the only "problem" is in your mind.
I'm not sure how it happens or why some people are "materialistic", and some are not. I think part of it is just how you're wired, but for me I think growing up poor had a lot to do with it too.
I have very early memories of thinking "when I grow up I'm going to make a lot of money no matter what so I don't have to live like this." Of course, looking back, it wasn't THAT bad. We always had a roof over our heads, food on the table, and everyone was generally happy in my family.
Everyone except me, who it turns out was a very materialistic person.
Once I moved out on my own at 17 I went through a "partying" phase for about 5 years, but after that it was all business. Long story short I made a shitload of money and was living the life of my dreams.
Of course as most people find out, money can't buy happiness. I distinctly remember a period not long after where I was just sitting there in my mansion thinking "WTF? I have everything. I'm still not happy. Now what?"
It turns out that if your happiness is reliant on money, or even reaching goals, you'll never be truly happy. Because whenever you have a little success, you just move the goalposts that determine what success (and thus happiness) is.
I've since "downsized" twice. I thought I "got it" the first time, but it turns out I didn't truly understand or acknowledge the fact that my happiness was still somewhat based on material things and money. I downsized my life big time, and thought everything would be great, but a few years later I found myself in the same fucking boat as before.
Then I REALLY started to understand what being "happy" is all about, and I downsized AGAIN. I'm talking about living below my means to the point where I live like someone who makes at least 1/5 of what I make or could afford.
I learned some serious lessons the hard way. I consider myself lucky to be born with at least a bit of financial conservatism or I could have easily blown all my money and I'd be seriously fucked right now.
Anyway, if any of this sounds like you, you'll save yourself years (or possible a lifetime) of torment if you figure out how to deal with this issue sooner rather than later.
Hearing a guy named Shawn Achor speak is what first opened my eyes and got me thinking. Then from there it was pretty easy. As they say, once you go down the rabbit hole there's no going back.
Here's a video that sums up a big part of what's going on. He has lots more, and a book.
[ame=http://www.youtube.com/watch?v=Hr_LKrpsefg&feature=player_embedded]Shawn Achor: The happy secret to better work: TED TALKS: documentary,lecture,talk - YouTube[/ame]