Boon Reflection doesn't cause you to gain life. Rather, it causes life-gaining effects to have you gain more life.

The effects of multiple Boon Reflections are cumulative. For example, if you have three Boon Reflections on the battlefield, you'll gain eight times the original amount of life.

If an effect says to set your life total to a certain number, and that number is higher than your current life total, that effect will cause you to gain life equal to the difference. Boon Reflection will double that number. For example, if you have 13 life and Blessed Wind causes your life total to become 20, your life total will actually become 27.

In a Two-Headed Giant game, only Boon Reflection's controller is affected by it. If that player's teammate gains life, Boon Reflection will have no effect, even when that life gain is applied to the shared team life total.