Did you know this idiom originated as early as the 13th century?
Not surprising really, there definitely is some truth to it. What I love about this idiom is its versatility. It can be applied to so many different life experiences. For example, the idiom can be applied to friendships. If you don't make an effort to stay in contact with your friends, you find that you can easily forget about them. Maybe you're shaking your head 'no' thinking if two people are good friends, the love will always be there. Though I somewhat agree, I still think it doesn't hurt to keep up with your friends. A simple phone call/text once in a while to check up on their life, it shows that you care and you're thinking about them. Your friendship will always be there because of your history together, but what about the future? If they are people you really care about, it's about having a life filled with memories together. This applies to many other valuable things in life we need to recognize so we don't forget the importance of them.
On another note, the idiom can imply the process of moving on. If you are experiencing a separation (either with a lover, friend, job, family member,etc.), it is easier to let go and move on when the person/thing is out of sight. When you don't have a constant reminder of the memories you had with that person/thing, those memories will eventually just slip your mind. Every day life will be about the new experiences you create. So store away or throw out all those pictures, gifts, memories you have of that person/thing if you want to move on.
Basically, the important things in life are what's in front of you. You choose what's in front of you. So choose carefully.