It is funny how these things get started. I know someone who will not only NOT thank you, but she will ask you to turn your head while she 'steals' it.
Some of the other non-gardening superstitions that my Mother-in-law believed in were:
Never turn a loaf of bread upside down, because it represents the bread of life.
Never hang an article of clothing on a doorknob, because it represents a 'crepe' that they used to hang on the door when someone died.
Never put a new pair of shoes on the table, or they will hurt your feet.
I guess they didn't have television to entertain them, and they had to manufacture their own entertainment. LOL!!

That is the absolute truth, my Granny impressed it on me as a child! You should never thank someone for sharing plants with you, but you can always say "I appreciate that!"

Janie who would never be caught thanking anyone for plants!

When I thanked my neighbor for the plants she left with me when she was moving, she also said that was wrong. According to her, you're supposed to say "I'll give them a good home." Sandy

why, oh why, shouldn't we 'thank' someone for giving or sharing a piece of her garden with us? I just don't get it.
But neither do I get what my mom used to say to me, she said "sing before breakfast, cry before supper "
I never understood that as a child and I sure don't understand why I can't sing all day and night if I want to.
But then she told me her Dad told her as a child that "if she put salt on a birds tail, she'd be able to 'catch' him."
well, duh, I told her of course you'd be able to. If you got close enough to put salt on the poor birds tail, he'd be in your hands already!!
People say the strangest things.
DQ

I've also heard this saying all my life. If you tell someone "thank you" for giving you a plant, as the saying goes, the plant will die, or not grow as it should.

Another gardening superstition: In the day, every grandchild born was given a plant called Hens & Chicks by his Grandmother.

I don't know if this is a tale, but I always give plants to friends because if yours should die, you have someone to get it back from. This certainly has worked for me....can't tell you how many plants I have lost and had to beg seeds or cuttings back from a friend.

Like a lot of old superstitions, I think it is a way of "averting the devil's attention".

If he hears you say "thank you" he knows you've been given something you like and he'll set about killing it!