From an article by Derek Sivers on the benefits of a daily diary:

“Years from now you might be looking back, wondering if you were as happy or as sad as you remember during this time. […] We so often make big decisions in life based on predictions of how we think we’ll feel in the future, or what we’ll want. Your past self is your best indicator of how you actually felt in similar situations. So it helps to have an accurate picture of your past. […] You can’t trust distant memories. But you can trust your daily diary. It’s the best indicator to your future self (and maybe descendants) of what was really going on in your life at this time.”