I'm not a believer in any sort of divine or spiritual element to karma, but I do believe it happens.
I believe you attract to you a reflection of what you are.
I think that when a person behaves well, is generous and considerate, others are more likely to respond in kind. People are more inclined to help someone when they believe their help is appreciated. When someone habitually takes advantage of others, is rude, agressive, mean or just not pleasant to be around, it bites them in the a**.
"There is no great genius without a tincture of madness." - Seneca