From the Super Mario Wiki
The player plucks a petal off a flower (which resembles the Smile Meter from Yoshi's Story), using their stylus, and the screen will say "Loves Me". After they pluck another one it will say "Loves Me Not". This continues until the player has plucked the last petal. The amount of petals on the flower varies, and rarely, there will be only one or two petals. If it ends with "Loves Me", a wonderful, musical glockenspiel will sound. If it ends with "Loves Me Not", a sad cat meow and a goat bleating sound will play. Three "Loves Mes" in a row will result in "Really Loves Me!", resulting with a happier glockenspiel. Three "Loves Me Nots" will result in "Really Loves Me... Not!", resulting with very unhappy sounds. On even rarer occasions, all the petals will fall off immediately, which usually occurs after plucking a flower with only one or two petals. If this happens, the "Loves Me Not" sounds will play, but the screen won't say "Loves Me Not".
Depending on what result the player gets, Yoshi will react differently on the top screen. If the player receives a "Loves Me" Yoshi will be seen raising his arms in victory. If the player receives a "Loves Me Not", Yoshi will look down and be upset.
The New Super Mario Bros. version of the game can be played against an opponent. Each player can pluck 1-3 petals at a time, with the goal being either to pluck the last petal if the result will be "Loves Me" or to force the other player to pick the last petal if the result will be "Loves Me Not". "Really Loves Me!" gives 1 point and "Really Loves Me... Not!" takes 1 point. Player must pluck at least one petal on their turn. This can be 3, 5, or 7 rounds. Most points after that many rounds wins. Randomly chooses who goes first each round. All play in order. When the game ends, Yoshi will react, much like the Super Mario 64 DS version. The winner can see Yoshi raising his arms in victory, while the losers can see Yoshi looking down, upset.
Play this minigame for advice on your love life! Pluck the petals to find out how your crush feels about you.