The virtues, talents, accomplishments, and possessions of others make us love and esteem them: Because these objects excite a pleasant sensation, which is related to love; and having also a relation or connexion with the person, this union of ideas forwards the union of sentiments, according to the foregoing reasoning.
Hume summarizes his principle of association of ideas (and so the passions operate): what is pleasant causes pleasure, and it is through the relation to the Self (of belonging, of property that such thing becomes even more pleasant.
Virtue naturally pleases us:
A generous and noble character affords a satisfaction even in the survey; and when presented to us, tho' only in a poem or fable, never fails to charm and delight us.
The talents and qualities, especially when less common, have even greater value in a person we seek a relationship with. Objects considered as beautiful, yet onerous, bring certain benefits when at the disposal of the person to which they belong (for instance, this person may decide to wear elegant clothes when they wish, even emphasize on some of their physical perfections).
These are not causes per se; but since these pleasant things related to the person whom we seek a relationship with, they participate to the amorous passion.