Also, a romantic love can ruin a friendship, so it can be better to hide it.
And sometimes you're not sure if you feel love (platonic or romantic one), or not. So you technically hide it if you actually feel it but aren't certain about it.
Hatred has been the driving force for many of our historical events. It's not difficult to hate.
It's probably even easier to spread hate nowadays than it used to be - just be anonymous, and what can happen to you, right?
It's also easy to hate if you've lived in the same place your entire life, with the same people, same culture, belief system and so on. It's the only thing you know. Everything else is bad. Or that's what you're taught, at least.
It's common to be hateful if you've got self-image issues, too. If you hate on other people, you'll feel better about yourself or forget about what you dislike/hate about yourself.
It's so much easier to be mean and hateful than kind and loving. What do you need to hate? Only anger, probably. You don't even need a strong argument, you can hate on literally anything or anyone for literally anything. It doesn't even have to make sense. What do you need to be kind and show love? Be self-aware, patient, empathetic, educated, actually taught how to express love. Those aren't qualities every person has or learns/is taught to have.
All of these reasons are just the ones I could think of at the moment, surely there are tens of other ones as well.
A nice question, by the way.