Gryffindor: short and to the point- it says what they mean and are open and honest. They do not shroud their feelings with fancy language

Ravenclaw: It is poetic, because they want their lover to feel the beauty that they see in them. They make comparisons, and list the reasons they love them

Hufflepuff: A reminder of their love, and a promise to keep them in their hearts forever, and stand together through anything that may face them

Slytherin: it is meant to build upon their relationship, rather than establish or remind. It speaks of plans, promises, and memories with the person