I don't control life, but I can control how I react to it
Macklemore believes that the key to happiness in life is not what happens to you, because that is completely out of your control, but rather how you react to what happens to you and how you deal with any given situation, because that is all that you can really hope to control in life.
Perhaps he means that some influences, good or bad, appear before him without him asking for them. But, he is his own sentinel to act accordingly when confronted by an influence. There is no point in getting upset about certain things that are out of his control.
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