"Trees lose their leaves because they become a draw on the energy of the tree, as the tree would otherwise have to feed the leaves through the winter and autumn." That is the Wikipedia reason for why deciduous trees lose their leaves in the autumn.
One of the most life changing truths in the Word of God to me has been the truth about the necessity to renew our mind...that through doing that, with the washing of God's Word, our soul, (mind, will and emotions), is restored.
Our old ways of thinking, reacting, responding, are like autumn leaves...life sucking, life draining... In order to survive and thrive during the dark, cold winters in our lives, those things have got to go in order for the new to come.
When we are born again we become new creations in Christ. But like a new computer loaded with old software, our minds are still loaded with all of the old experiences and thoughts and attitudes of our before Christ life. Those have got to go, be deleted, allowed to fall away...or they become life draining lies. When winter comes, they suck away at the life sustaining flow in us by raising up doubts about the reality of our salvation, of God's love and grace, of His goodness.
Just like the new leaves of spring, the new has got to replace the old...the truth of God's Word, replacing all the lies our minds have been filled with. Ephesians 4:23-23 Amplified Bible "...put off the old unrenewed self...be constantly renewed in the spirit of your mind, having a fresh mental and spiritual attitude, and put on the new nature, created in God's image."