Shrek's love for Fiona is founded on friendship. After he rescues her from the tall tower, Shrek realizes that even though Fiona is a beautiful princess and he is an ogre, that they can be friends and have common interests. 

Lord Farquaad, on the other hand, knows nothing of Fiona's true nature or personality.  He chooses her from the Magic Mirror based on appearances alone.  His interest in her is shallow, because he does not even consider what she might want from a relationship.  He selfishly fixates on his interests and his needs. 

Fiona and Shrek's relationship is more valid, because it is founded on mutual love and respect.