You know the problem. A lovely tune - but later somebody turned it into a song; and now, whenever you hear the tune you simply can't help but hear the words as well...
And when the words are sentimental tosh - well, that makes it even worse.
It's a rare and difficult feat to write a lovely melody - so having it ruined thus must be exceptionally irksome.
Example one is the Londonderry Air:
Or Brahm's Lullaby - really beautiful:
One the other hand - sometimes the song is So Good; that it must be forgiven. The finale from Mozart's Horn Concerto K495 is certainly a joyous piece:
But the comic song devised by Flanders and Swann is so utterly brilliant that maybe it even adds to the enjoyment: