Could probably because you're too kind. You can not refrain from helping people who ask for your help. Be kind to those who really deserve kindness because they have big heart to help you or others when needed, but be firm and assertive to those who's manipulative. In one book "the Art of Manipulation"(R. B. Sparkman), it was taught to always trade your kindness with those manipulators, never ever give your kindness freely to manupulators. That way, they'll respect you instead of thinking of you as an easy prey to be taken advantage of. When they persistently ask for your kindness, tell them to pay an equal amount of your kindness, no need to be in the form of money, but something like painting your house or if you can, make them pay more than what they get from you. Use time delay tactics, don't help immediately even if you can. Make them wait. The key point is: meke'em pay, then they'll respect you and leave you alone if they are not physically thuggish enough.

Being kind doesn't always mean to be helpful all the time. I used to have this wrong concept of kindness. Letting yourself to be abused is not kindness, it is encouraging others to be abuser.