I don't. I consistently put others before myself and go out of my way to help people, even strangers. I don't do it for any reward like karma, I just can't bear the thought of someone needing help and not getting it. But life is more often than not a struggle for me.
Examples of why I don't believe...
i. Just after Christmas, a lady I didn't know posted on a FB group that she had a pigeon in her garden unable to fly and asked what to do. When she learned the nearest animal sanctuary was an hour's round trip, she was reluctant to take it there. I ended up taking it there and two weeks ago the sanctuary rang to say it was better and ready to come back to my town and be released.
While I was inside the sanctuary, someone drove into my parked car and drove away!!! Gee, thanks karma!!
ii. My abusive arsehole of an ex-stepdad, beat me black and blue from when I was aged 6 to 16. I ended up having a breakdown at 16 and spent a decade on Prozac, then years of counselling to get over the abuse. Essentially having 25 years of my life marred by him. Police couldn't do anything when I reported him for historical abuse and he's lived the life of Riley, loads of money, and a long life, he's 80 this year. The wanker deserves to rot.
So no, I don't believe in karma.