It’s a sad song about the heartbreak of unrequited love.
not returned in kind
Unrequited is commonly used in relation to unreturned love. It is used more commonly by far than requite or it’s other derived forms. Requite means: make repayment for or return something.
When not referring to returned feelings or other senses of return, requite typically indicates a suitable non-cash repayment. To requite a wrong is to perform an act of revenge. To requite a kindness is to return a good deed.
Standard prefix: Note that when "un" is used as a prefix, it often means "not".