What the reasons for dropping my battery health and how can I stop to dropping it?

Every time you charge a rechargeable battery, it's battery health will decline little by little. This is what batteries do. ALL BATTERIES. The simple answer to your question is there is NO WAY to stop battery health decline. Your iPhone battery will continue to decline in its ability to hold the same charge it did when it was new and when battery health declines to 80%, it will be time to pay Apple to replace the battery.

Read this link about Maximizing Battery Performance, but remember there is NO WAY to stop battery health decline. None.

Batteries - Maximizing Performance - Apple