I see it all the time - you get with someone who has past hurts and baggage and you love them SO MUCH that you think you can love them through it. I see it a lot with women, since I feel our natural inclination is to be nurturers, but I've also seen men do the same (with a woman, or in relationships with other men). The issues can come from that person's childhood or their experiences in romantic relationships.
There's two ways you try to handle it -
The first way:
Heal them with your love. They've been cheated on and they have trust issues, so you can show them true commitment and honesty. They've been abused in the past, so you're going to show them how it feels to be loved by someone gentle; you would never talk down to them or put your hands on them. They were never told "I Love You" enough, so you're gonna say it everyday, every night. Whatever the issue, you're there to help.
Or, the second way:
