One Wish Willow is extremely specific and most importantly - personal.

It's not evil, it's not like haha you wish for that? I'm gonna fuck you up and it was confirmed but without confirmation we can clearly see that worker at the store seemed to be fine and that billion dollars just came from the sky.

Now Bear's wish was not only immoral but extremely sick, the entire nature of it but that's not the only reason why Nikki went batshit crazy. It is because whatever Bear felt for Nikki wasn't love, it was obsession and obsession meant love for him. He didn't wish Nikki to be crazy for him or be OBSESSED with him, he just wished for her to love him more than anybody, you can love somebody more than anybody but not go crazy, Willow just granted Bear HIS PERSONAL WISH, his understanding of what love is. wish Nikki is what Bear would've been in a relationship. He's insecure, unsure of himself, a lonely, selfish person with 0 accountability, his entire personality was that he wanted Nikki because he had an obsession about the idea of her, he didn't even know her, didn't even understand her.

Love can be anything, it's extremely personal and subjective and the wish gave him his understanding of it and that's why wish Nikki's love language was obsession and insecurity.