Why is it perceived as weak if you're kind to others ?

Who knows ... it takes a strong person to be kind and loving to others !!!

I think there are people who genuinely are kind because they are able the see the good in others. So, I'm gonna refer to these and not to some people who pretend to be kind or nice as a way to get what they want from others. Conciously or unconciously.

I think this kind of people are seen as coward. They are ask for reacting, defend I don't know what principles, enter into conflicts. It seems as you didn't have a strong personality if you behave in a different way. They see you as submissive. When they perhaps are only giving small things a relative importance.

I think because sometimes people think that means others can use you (ie favors).

its just peoples negative reactions

Because in a group it becomes obvious who got their way using the other person's kindness but 9/10 times it helps avoid too much unnecessary conflict.

It's only called weakness by those bullies, because it makes them so obviously look bad. It actually takes great strength to be kind.

only by unkind. cold, unhappy people

It's unAmerican!

It's not

yes right....

Vulnerability

Only to stupid ignorant people it is seen that way..