What if they stole from you to help someone else? Shouldn't you ask why?
The person doesn't have to understand, the person who stole has to try to make amends that's it. That person understanding or forgiving the theft is totally out of thief's control. If the thief is making amends for any other reason, then they are not making it for the right reason.
I've rarely been able to forgive straight away especially if the behaviour has caused an enormous amount of harm, I think time plays a role, and that cannot be rushed or hurried, time creates other experiences in life that may help a person to understand past wrongdoings of others, but the most important thing is for the person to not be consumed by the harm that person caused because then they've won. However, understanding and explanations may help, other times they don't. What one person may think is reasonable, will not be reasonable to another.