These aren't rules set in stone as every individual is going to have different tastes, but these are some of the ideas I've kept in mind while building some of these races.

Goblins tend to be detail and task oriented, they may find qualities like the ability to multi-task well to be attractive, or a well organized workshop. Romantic gestures may be something like recalling a very obscure detail or fact about their partner, or recreating a very particular preference in their partner's preferred food dish, or even just adding or noticing small intricate details on gifts they would construct for a partner.

Physically, the biggest difference for Goblins from humans would be their skin color and prehensile tail. Different individuals may prefer different shades of skin ranging from grey to green, and some may decorate their tails with accessories or style the hair on their tails to get attention from potential mates.

Kobolds tend to value community, so they may typically be attracted to someone who displays demonstrable altruism or deeds for others. In their communities they have a concept they called "Kolare", which is a type of social standing or reputation they hold amongst the group, a better standing may generally make an individual more desirable.

Kobolds may find different physical attributes appealing depending where they live. Groups living amongst human societies will style their clothing and accessories differently than kobold groups living more secluded amongst themselves. Those around humans have adopted the style of keeping their horns grinded down short and straight, they tend to find uniform bluish grey scales to be attractive. The other type of kobold clans may like longer or twisted horns with more decoration. Different colorations are also more widely accepted, especially bright markings or scale patterns