Complicated topic ... I understand the idea that a trait can only be remarkable if the opposite exists. But in case of love the question is if hatred is the opposite of love? To know that I love someone I need not to hate someone else (to be able to feel the difference). I just claim the opposite of love is indifference. So I can know what love is because I also know that I don't have deep feelings for others ... So actually I would not necessarily argue against 'your' two-side-theory, but I would argue about what the opposite of love is. :)

If it is objectively true to eat no meat ... we will never know. Everybody has his own subjective (and possibly well founded) 'truth', but that depends on so many things, ones gustatory sense, environmental and health related aspects, our own moral values and believes, ... right up to the question why the universe exists :-) , so that I guess there will never be an agreement (or at least not intermediate term) about the objective truth.
Do you think in case atheists are objectively right that God doesn't exist they should do everything to convince everybody about it? Actually I see it like that, that even if I am convinced (or strongly believe) to be right I respect that other people follow other ways of life than I do ... To tolerate even 'wrong' opinions could sometimes be objectively better than always fighting against them. :)
And concerning vegans again: I fully respect them, even if I don't belong to them. Not everything is about morally superior or not. I just decided to eat meat from time to time, but that's a personal decision, and I really see no reason to think it was better in general than deciding not to eat meat ...