
Not so she will be with him, but so her freedom of choice will not be compromised by the love potion. Shrek has a giant gingerbread man baked and storms the castle for Fiona.
SHROOK AND DONKEY CARRIGE FREE
This breach of her free will is what moves him into action.

He sees from their conversation how much Fiona hates Prince Charming and he feels guilty. The King prepares a tray with some tea, placing the potion in Fiona's cup before going to have a talk with her.

The Fairy Godmother orders Harold to slip Fiona a love potion so that she'll fall in love with her son, Prince Charming.You can tell by the look on his face after he decides at the last second not to give Fiona the tainted cup of tea that he knows this will cost him greatly. This shows that Harold really does love his daughter and doesn't want to force her into anything that she doesn't want to be in. After Fairy Godmother orders him to give Fiona a potion to make her fall in love with Prince Charming, he refuses albeit understandably afraid for his life. He sees how unhappy Fiona is with Charming and tries to talk Charming and the Fairy Godmother out of it, not concerned with what will happen to him. Despite his strong opposition to Fiona's marriage to Shrek, Harold starts to let go of this when he sees how genuine Fiona's feelings are for Shrek.
SHROOK AND DONKEY CARRIGE MOVIE
The heartwarming of this moment even better once you realise it's a Call-Back to a moment earlier in the movie when Puss threw a Stealth Insult at Donkey by saying he "still looks like an ass to " after drinking the potion.Noticing how disappointed Donkey is once he's changed back, Shrek turns to him and says: " Hey, you still look like a noble steed to me." Shrek and Fiona are happy to skip out on the Happily Ever After potion and remain ogres, but Donkey doesn't have a choice and doesn't get to stay a stallion.They are two people who never intended to fall in love with each other but true love found them anyways.

This song suits Shrek and Fiona's relationship perfectly.
