Can favored enemy be something like significant study of the favored enemy instead of a hate for them? For example a gnomish ranger that studies constructs with favored enemy construct, or a field medic with favored enemy human treating mainly humans?

Yes.

FE simply represents an affinity and/or especial effectiveness fighting that enemy. Hate need not be a factor.

However, do consider what prompts your character to study a given race. It's awfully convenient, for lack of a better word, to just pick FE (Human) if there's a chance you'll be getting in PvP.