Past lives should never serve as justification for shitty behavior : "Oh i'm a jerk because this happened in my past life." Dude, like… that's not even close to how it works. You're a jerk because you are the product of your upbringing and the sum of all the traumatic events you have been subjected to since your conception.
HOWEVER our past lives do influence our current lives to some extent. Think of it like a network of trees. The soil is your soul. The roots grow within the soil, connecting with each other underground. All trees are attached together but you can't tell by looking at them: they all seem to be their own individual tree. Each tree is one life. They are separate but connected. They belong with each other, each one giving birth to the next; each one participating to the growth of the others.
We hold within our souls the memory of all our incarnations. It's a well of wisdom and experience we continuously draw from, without knowing. Reincarnation is a game of connect-the-dots. We do not get better, we get fuller by connecting more and more dots as we go and remember our essence.
Our past lives tell the story of our growth and there is no going backwards. We cannot get worse, either. Just because you were a saint in one life and Hitler in the next doesn't mean you fucked up. Every incarnation is a choice on behalf of your soul. We choose the family we incarnate into. Yes, we choose our parents! So much for blaming your dad for everything, huh? (I know, I've done it). Our soul wants to experience everything on the spectrum of feeling because it participates to its growth and it learns the most by way of contrasting experiences. That's why we go through so damn much. But remember, life will never give you something you can't handle.