Why is it necessary to "translate" songs to Mandarin that are written in Standard Chinese, but originally sung in Cantonese?
For your example, it is more like 2 different songs based on the same rhythm rather than translation, because the meaning of the lyrics are far from each other. And for the reasons we redo the lyrics in Mandarin:
a) Sometimes, the pronunciation of the original lyrics in Mandarin doesn't fit the rhythm.
b) Some of the expressions used in Cantonese are not commonly used in Mandarin, like in this example: "愿再可", "你共我" are not standard expressions in Mandarin.