According to Shailendra (1923-1966):
"sweetest songs are those that we sing in sad voice"
"hain sabase madhur wo geet jinhen, ham dard ke sur men gaate hain,
ham dard ke sur men gaate hain;
jab had se guzar jaati hai khushi, aansoo bhi chhalak te aate hain,
aansoo bhi chhalak te aate hain;
hain sab se madhur wo geet..."
(Patita, 1953, music: Shankar Jaikishan शंकर-जयकिशन)
According to Percy Bysshe Shelley (1792-1822):
"We look before and after,
And pine for what is not:
Our sincerest laughter
With some pain is fraught;
Our sweetest songs are those that tell of saddest thought."
Notice Shelley died even yonger than Shailendra.
Their songs remain sweet as ever.