People are generally made up of the past, present and future and loving someone means you have to accept them in totality. Knowing thier past and accepting whatever happened there as part of your them, creating the present with them and helping them shape the future.
Not everyone is proud of their past, some people have done so many crazy and unpleasant things they rather not tell. Some other persons have been very unfortunate and have been victims of circumstances. Some got deceived for thier naivety, used, got pregnant and dumped. Some are victims of rape, physical abuse and assault which may be presently telling on thier social and psychological balance.
However the past is, if you genuinely love someone and want a true relationship with them, the present is what you should be concerned about. What they are doing currently and how they are living their life presently is what you should know.
If you met them trying to fix thier lives and right thier wrongs, irrespective of what they may have done in the past, you should take them for who they are and see how you can help. If they have been victims and trying to pick up thier pieces, help them in whatever way you can and show them that they can still be loved right.
A true relationship is that in which partners accepts thier past, supports thier present, encourages and helps build the future together. Do not judge someone trying to fix thier life. We all make mistakes and we all have a past. True love never judge, neither does it hold grudges. All they owe you is the truth and all you have to do, seeing the sincerity in them, is to help them forget the past and become a better person, you will be proud of the person they will become tomorrow.