Q: If one says “Your thoughts are welcomed,” why does one respond to “Thank you” with “You’re welcome,” not “You’re welcomed”?
A: In the sentences “Your thoughts are welcomed” and “You’re welcome,” the word “welcome” is being used in two different ways, as a verb in the first one and as an adjective in the second.
As a verb, “welcome” means to greet cordially or accept with pleasure. You might ask your doctor, for instance, “Do you welcome new patients,” and she might reply, “Yes, I welcome them” or “Yes, new patients are welcomed.”
Similarly, when you say, “Your thoughts are welcomed,” you’re using “welcome” as a verb (a past participle in this case).
On the other hand, in sentences like “I felt welcome” or “He’s welcome to visit” or “The rain was welcome” or “She gave welcome advice,” the word is an adjective meaning received gladly or giving pleasure.
It’s this adjectival sense that we use when we say “You’re welcome” in reply to “Thank you.”
Dictionaries don’t usually define the adjective “welcome” in this idiomatic usage. The Oxford English Dictionary, for example, describes “You’re welcome” simply as “a polite formula used in response to an expression of thanks.”
In case you missed it, a recent item on The Grammarphobia Blog discussed the history of “You’re welcome.”