Why do I keep having nightmares about a person from my past?

Depending what their involvement in your life was, it's still affecting you and you haven't gotten over it

The nightmares maybe because that person had a great impact on your life in the past and your mind wants to resolve it

It's because you cannot let go of that person or what they did to you. My father was cruel to me and till this day, I get nightmares about him (although we haven't seen each other or spoken since 2012). I realized that I had bottled my anger and fearfulness. I spoke about it with my friends and it helped me considerably. To put it in the concept of Sigmund Freud, the nightmares are the release of the unconscious mind into the conscious mind. You need to let go of the person. You need to forgive and forget them. Repetitive nightmares are an indication of an unresolved or inharmonious mental state. If you wanna go, let go.

that is deep, it can have many many reasons since dreams tell a lot about our subconsciousness ad the things that we though we forgot. So it is really important how you see that person in your nightmares. if you want to talk about it, i can listen to you.

People who constantly appear in our dreams tend to be consciously/subconsciously on our minds when we're awake. Are there things you are really afraid of about your relationship with that person? Or regrets you have?

I understand your problem. There are thoughts which haunt us and every night is not peaceful. I might not be correct but maybe your experience with that person was traumatic. Dream is said to be the reflection of our thoughts. Maybe, deep down that memory has left a scar in your heart. I feel that if this disrupts your daily routine or affects your health in any way, you should seek professional help. Sleep is important for daily functioning and your mind needs rest. Fee free to consult and it is completely okay to do so. I hope this will help you!

Your mind unconsciously thinking about them or you have not let them go and you constantly thinking about them.

Perhaps there is some unfinished business, some unresolved feelings, some fear or anxiety, some discomfort overall; maybe he or she is on your mind because of positive things, because its necessary to consider them, to pursue contact again, to let emotions be released, to let go! It may be time to move on, to come to terms with certain circumstances; take a deep breath.

The subconscious looks for reason... Dreams are also a tool of the mind that serves as a testing ground to hash out our true feelings by making situations that can not exist in real life.

Because that person affected your mind in such a way that you're having a hard time letting go. That person still resides in your sub conscious mind.