When you want what is best for someone you love - you are willing to let them go. Letting go of somene will give them an opportunity to grow. It may hurt to let someone go that you love, but it probably best for you too!
If you are speaking of children - they must grow up.
If you are speaking of a relationship - the person and you must mature.
We let go because of bad memories; the relationship is not going anywhere; we need to move on with our lives and let someone else in.
We might not understand it at first - but time will heal!
Lastly, if you (we) have hurt someone, letting them go may be the answer to a better life for all involved.