WinterSnowfall: Depends of course how one defines success, but yes, personal success is important to one's well-being. This, for some, may mean climbing the Everest once a year, while for others it may mean the chance to enjoy a nice cup of tea on the porch every evening. I personally see it as "being where you want to be in life", an ever moving goal post, not excluding the possibility of deluding oneself in certain cases. Ergo "success" can only be as truthful and genuine as its bearer.
^Pretty much this.
Two good ways to become unhappy:
1) Setting your sights too high.
Nothing wrong with a challenge, but it should be realistic. To reach a challenging goal it's better to map out smaller steps first that are easily achievable. If you fail at a big, daunting challenge, it can be frustrating and de-motivating. But smaller steps keep up a constant flow of little successes.
2) Related to that: Never look back.
I think that's a common problem. Instead of looking back and cherishing what one already achieved, and being happy about it, there's immediately the next thing that needs to be done. If this happens with tasks "from the outside" (ie. swamped at work) that's a good way into burn-out and depression. But it can also happen when you're driven by your own goals without ever stopping.
It's really important to look back occasionally, and to actually take in the successes you had. Otherwise they can't do anything for you. And sometimes it may be worth considering that "It's enough, actually. Time for tea on the porch, fuck the Everest.". :-)