I have had many best days.
Days filled with runs, hiking, kayaking, friends quality time, lovers, biking, being at ease enjoying a beautiful day, days when my senses were so alert everything was divine and transcendent. The sounds were embued with warmth and everything had a meaning or meant nothing at all and was just mine for the taking.
The reason you don’t remember your best days is because your body is relaxed instead of being on high alert due to stress or fear. These times impress strong memories and often triggered your mind goes back there making those memories even stronger.
If you want to remember the great days. Then write them down talk about them think about them. Access them.. they will become stronger.
There doesn’t have to be a why. Don’t complicate a simple thing. Memories either make you feel good or make you feel bad or conflicted.