You could make sure they are awarded for giving likes by awarding badges. There are a few default badges related to likes, for example the ‘Higher Love’ and the ‘Gives Back’ badges. You could go a step further and create custom badges. Make sure to enable the Allow badge to be used as a title and Show badge on the public badges page settings, to make the badges are more visible.
Another thing you could do to encourage liking, is run a query periodically to get the users that gave the most amount of likes and add them to a custom group. You could then configure a flair to this group, so they have recognition accross your forum as “likers”.
Think of a flair like this:
