Was ich nicht weiß, macht mich nicht heiß

Meaning: What you don't know can't upset you.

Literally: What I don't know doesn't make me hot

Example: “Erzähl mir nicht, was es gekostet hat. Was ich nicht weiß, macht mich nicht heiß.” — Don't tell me what it cost: what the eye doesn't see, the heart doesn't grieve over.

Equivalents in English, Spanish, Italian, Portuguese, Dutch, and Polish

English: What the eye doesn't see, the heart doesn't grieve over
Spanish: Ojos que no ven, corazón que no siente (literally: Eyes that don't see, heart that doesn't feel)
Italian: Occhio non vede, cuore non duole (literally: The eye doesn't see, the heart doesn't ache)
Portuguese: O que os olhos não veem, o coração não sente (literally: What the eyes don't see, the heart doesn't feel)
Dutch: Wat niet weet, wat niet deert (literally: What doesn't know doesn't hurt)
Polish: Czego oczy nie widzą, tego sercu nie żal (literally: What the eyes don't see, the heart doesn't regret)