Don't trust the person who has broken faith once. - William Shakespeare

Neither a borrower, nor a lender be;
For loan oft loses both itself and friend,
And borrowing dulls the edge of husbandry.
This above all: to thine own self be true, 85
And it must follow, as the night the day,
Thou canst not then be false to any man.
Farewell; my blessing season this in thee!

This is the excellent foppery of the world, that when we are sick in fortune (often the surfeits of our own behavior) we make guilty of our disasters the sun, the moon, and stars, as if we were villains on necessity; fools by heavenly compulsion; knaves, thieves, and treachers by spherical predominance; drunkards, liars, and adulterers by an enforced obedience of planetary influence; and all that we are evil in, by a divine thrusting on. An admirable evasion of whoremaster man, to lay his goatish disposition on the charge of a star! My father compounded with my mother under the Dragon's tail, and my nativity was under Ursa Major, so that it follows I am rough and lecherous. I should have been that I am, had the maidenliest star in the firmament twinkled on my bastardizing.

I must be cruel only to be kind;
Thus bad begins, and worse remains behind.