Robert A. Heinlein Unbearable Quotations

Jealousy is a disease, love is a healthy condition. The immature mind often mistakes one for the other, or assumes that the greater the love, the greater the jealousy -- in fact, they're almost incompatible; one emotion hardly leaves room for the other. Both at once can produce unbearable turmoil...

When I don’t understand, I have an unbearable itch to know why.