Marriage Requirements in Rune Factory Frontier include the usual general requirements of expanding your house so that you can purchase a Double Bed for the Bedroom, as well as the special Proposal Item in the form of the Wedding Bouquet that is the equivalent of the Blue Feather in other games.
There are requirements that are specific to each Girl as well. To marry any Eligible Girl, you first must complete her 'Story' by experiencing a sequence of Dialogues and Events.
In Anette's case, you will have forewarning of the shape of things to come when you receive letters she wrote long ago 'to the Stars'. The following video shows the sequence of Events that allows Anette to realise her dreams: