1) Arnold never said why he defected, so we have to try to draw inferences. Being passed over, being stiffed, and being prodded by his Loyalist wife are obvious factors pushing him in that direction. Soon enough, he had a British generalship and significant command.
2) That phrase is from Locke, and it was used by Mason. Forests of trees have been spent trying to answer why Jefferson substituted “the pursuit of happiness,” and I’ve never been completely persuaded that anyone had given a convincing answer.