Because kindness is something, you want to spread. Be kind even to those who aren’t to you.
We can either be unkind and stay unhappy while making others unhappy, or we can be kind and happy, spreading happiness around.
If someone is unkind, it’s their internal struggle making them do so. It has nothing to do with others, so be compassionate to them and give kindness in return. You never know; your gesture can change them for good.