It's simple as that. Realists believe interests drive states to cooperate with others, liberals also tend to think the same, with a different approach to understanding of cooperation as in win-win benefit for both countries, rather than relative benefit or zero-sum gain from a realist perspective. It's all about interests, friendship is just an empty word used in public.
Also, even if you want a "friendly" and "brotherly" good relations with other regional or global actors, well, you have to make sure that they also want the same thing. If they don't, you can't blame others for not having good relations.