when you have to be cruel to be kind, it simply means you are dishing out tough love to your lover. (S)he will later recognize that you have his/her best interests in mind and will later be grateful for what you are doing. Things will all pan out in the end, your lover just has to trust you.
"Cruel to be Kind" is a late 1979 hit for British rock'n'roller Nick Lowe. He's made other great witty songs as well, check out his 1989 greatest hits release "Basher: The Best of Nick Lowe", it's terrific. Also, a different song of the same title appears on the debut album "Resident Alien" by Spacehog, from 1995.
... cruel to be kind
in the right measure
cruel to be kind
it's a very good sign (dit dit)
cruel to be kind
means that I love you
babyyyy
you gotta be cruel, you gotta be cruel to be kind
woo hoo hoo hoooo hooo hoooo...

by I Saw U2 Live Twice June 3, 2007
Get the cruel to be kind mug.