Today's proverb is often used in Swahili to describe a person who makes a mistake that could have been foreseen and suffers negative consequences... like the truck driver in this picture from Oxfordshire, UK. Regardless of what your parents taught you (or failed to teach you), you will eventually have to confront the harsh realities of life and learn from experience.
See also: If a child cries for a razor, give it to him (Mtoto akilia wembe, mpe)

He who is not taught by his parents is taught by the world. (Asofunzwa na wazazi, hufunzwa na ulimwengu)
Here's a poem by the poet Akilimali Snow-White about this proverb. (My translation from the original Swahili)

In the age they fooled me, my old folks in raising me,
I failed to learn the new movements of the world.
Today I please myself, to the people of the world, listen:
He who is not taught by his parents, is taught by the world.

I couldn’t have done any work without humbling myself before them,
Obeying to flatter them, then to serve them,
Even when I pleased them, they taught me with intention,
He who is not taught by his parents, is taught by the world.

Now I can speak European languages without difficulty,
like English and others too,
With effort I learn, and even they have raised me.
He who is not taught by his parents, is taught by the world.

I can converse without blemish,
And lead amidst evil, removing the blemish,
In the end the place pleases, one step towards harmony,
He who is not taught by his parents, is taught by the world.

There is nowhere I have overlooked, without investigation,
All sides examined, knowledge I have taken,
I even know how to sell products and buy,
He who is not taught by his parents, is taught by the world.

The amount which I have learned, not a little by fumbling,
I am pleasing where I come from, I employ good work
It’s hard to scorn, how it raises me,
He who is not taught by his parents, is taught by the world.

It’s not right to ignore what you don’t know
Try to investigate, and then analyze,
When your intention is tightened, you can’t fail to know a thing,
He who is not taught by his parents, is taught by the world.

The tasks I taught myself, my father didn’t know
He didn’t know English, or selling and buying,
but only praising oneself, that was when I, the child, knew,
He who is not taught by his parents, is taught by the world. 

I give more for you, you all who helped me,
All of you who’ve taught me, Lord give you health
God fill you all with happiness, and return goodness to you,
He who is not taught by his parents, is taught by the people of the world.
- Diwani ya Akilimali

What do you think about this poem? What does it mean? Can you improve the translation?

Fikeni E. M. K. Senkoro (1988) wrote of this poem (my translation):

As the appetite increases, food tastes better.

The proverb first appeared in Miguel de Cervantes' Don Quixote, published in 1615 (in Part II, Chapter V)

Parents often say this to their children when they are fussy eaters.

Watu wengi wanaogopa kuuliza maswali kwa sababu wanahofia kuonwa mjinga. Lakini kuuliza maswali ni njia bora ya kujifunza kutoka kwa wengine. 

Pia kuuliza maswali husaidia wenzako. Je umewahi kusita kuuliza swali kwani ulidhani wengine wameshaelewa... lakini baadaye uligundua hawakuelewa pia? 

Kiingereza

This proverb refers to the tendency to imagine or dwell on the notion that other people have nicer things...like your neighbor having a nicer lawn. 

The original source of this proverb is Ovid's "Art of Love", a book of advice for men and women about finding and keeping romantic relationships, written over 2000 years ago, about 2 AD:

One lesson from this proverb is to focus more on what you have, ignoring what others may have that is better. Another lesson might be to put your attention on improving your own situation (like watering your lawn) rather than dwelling on the fact that your lawn (or situation in general) is inferior.

An interesting variation on this proverb is: "The grass is always greener where you water."

Similar proverbs from other cultures:

Hindi:

Chinese:

Japanese:

Russian: