Put together the couples in love with a piping path. Build a path from one girl to a boy when they are both potentially in love. You know they are because a big pink heart appears near those that are close to fall in love (while the rest just have a small orange heart). When you join a couple with a pipe, the big pink heart turns orange, and when there's no more pink hearts you pass to the next level. It will be more difficult as you progress: there's more pink hearts and you have less time.
Click on the central squares to rotate the pipes to make a path.