Peter Rukavina quotes the New York Tribune about how Anticipation is the real thief of joy (as opposed to comparison):
Anticipation is, in truth, the real thief of joy. The best times are always the unexpected ones. It is not the parties that you plan for weeks and look forward to that come off. There must be surprise and novelty and freshness to yield the last word in happiness and thrill. That we had on Thursday afternoon. The real thrill has passed, never to return.
While the piece is a musing on the pleasures of serendipity and random wandering, I'm reminded instead of anticipation's opposite, something I feel often as well: anxiety. Whereas anticipation is looking to the future awaiting something you are excited to happen, anxiety is looking to the future awaiting something you dread. And it robs you of joy as well, albeit in a different manner, consuming you with worry. The phrase "real thief of joy" implies that there can only be one such thief, but truthfully, there can be many, and among them, comparison, anticipation, and anxiety are probably the worst offenders.
It is of course human nature, and often prudent to look to the future, but it is best not to let either anticipation or anxiety overwhelm us. One should always remember to live in the present and not let the future overshadow what we have right in front of us.