It may sound obvious, but I've come to learn that regret is caused by a changing mindset. The mind has many states due to new experiences, changing environments, etc. In each state of mind, you'll have a different decision making process and be satisfied with completely different decisions. For example, someone who plays World of Warcraft 8 hours per day will be happy and satisfied to make the decision of not hanging out with friends or pursuing career ambitions. At the same time, careerist such as an investment banker will feel obligated to sacrifice leisure and family pursuits for job promotions.
So where does the regret part come in? In our youth, the state of mind is by no means stationary but is transient. When your state of mind changes (and thus your decision making process), you'll look back at past decisions made under a different mentality and regret them, since those decisions no longer make any sense to you. When we're young this doesn't really matter and should be accepted, but as one settles down and makes long-term decisions (such as marriage, decisions to move and switch career paths, etc.) it's important to stay within a consistent mindset and reduce any irrational wandering. Sometimes our state of mind may be temporarily affected by new experiences, such as influential words from a book we read or a compelling talk w/ a new friend. One should always be wary of these effects and stay clear-headed, while thinking independently.
As we ground ourselves and stabilize within certain bounds of a state of mind, regret will occur less frequently and we may even find some form of contentment.