This makes sense
Why are good, thank you, and you're welcome all related? That makes less sense to me
Discussion
I'm not familiar w/ why GOOD + THANK-YOU are so similar (or sometimes the same, with their meaning dependent on context)-
On a subjective level, the feeling/concept of GOOD/GRATITUDE share similarities, but I don't know the etymology w/those two signs to give a good answer
Interestingly, there's not *really* a sign for "YOU'RE WELCOME"; instead, the following signs are more frequently used: FINE++, SURE, NO-BIGGIE, "pashaw" (more of a gesture meaning 'no worries')...
There are many who do sign the literal "WELCOME" to mean "YOU'RE-WELCOME"... that's a linguistic pet peeve of mine as conceptually it doesn't make sense.