Trust not yourself; but your defects to know,
Make use of ev'ry friend—and ev'ry foe.

Even if you think you're doing well and have it all figured out, there is a voice you will always inevitably hear at some point which nags at you and says "but wait..." Don't ever dismiss it, listen to what it has to say. Life will never be close enough to perfect, and listening to that voice means stepping outside of yourself and considering your own wrongdoings and flaws.

The greatest magnifying glasses in the world are a man's own eyes when they look upon his own person

No man ever got very high by pulling other people down. The intelligent merchant does not knock his competitors. The sensible worker does not work those who who work with him. Don't knock your friends. Don't knock your enemies. Don't knock yourself.

Many men have been capable of doing a wise thing, more a cunning thing, but very few a generous thing.

All fame is foreign, but of true desert; Plays round the head, but comes not to the heart: One self approving hour whole years out-weighs Of stupid starers, and of loud huzzas; And more true joy Marcellus exil'd feels, Than Caesar with a senate at his heels.